US state department spokesman PJ Crowley has resigned after calling the treatment of the man accused of leaking secret cables to Wikileaks "stupid".
He said he was taking responsibility for the impact of his remarks about Bradley Manning.
Private Manning is being held in solitary confinement at a maximum security US military jail.
He has been on suicide watch at the Quantico marine base in Virginia and is shackled at all times.
He faces 34 charges relating to the leaking of 720,000 diplomatic and military documents.
Secretary of State Hillary Clinton said she accepted Mr Crowley's resignation "with regret".
She said he had served his nation "with distinction", "motivated by a deep devotion to public policy and public diplomacy".
'Ridiculous'
Intelligence analyst Bradley Manning served in Iraq
Mr Crowley was speaking to an audience at the Massachusetts Institute of Technology about new media and foreign policy when he made the controversial remarks.
He was asked by a participant about the "the elephant in the room" - Wikileaks - and, in the questioner's words, "torturing a prisoner in a military brig".
"I spent 26 years in the air force," Mr Crowley reportedly replied.
"What is happening to Manning is ridiculous, counterproductive and stupid, and I don't know why the DoD [Department of Defense] is doing it. Nevertheless, Manning is in the right place."
He said his comments were on the record, though he later added that they were his own opinion.
In his resignation letter he said: "Given the impact of my remarks, for which I take full responsibility, I have submitted my resignation."
It said he had been held "for 23 hours a day in a sparsely furnished solitary cell and deprived of a pillow, sheets, and personal possessions since July 2010".
He was also reportedly forced to disrobe on a daily basis.
The UK government has just weeks to convince EU officials that it will meet European clean air standards in London, if it is to avoid a court case.
The European Commission is assessing UK data for London and will decide in a month's time whether to give the UK an extension until mid-June to comply.
Tiny airborne particles, called PM10, are above EU limits in those two areas.
The UK is at stage two of a six-stage infringement process, EU Environment Commissioner Janez Potocnik told reporters in London on Thursday.
He said EU infringement procedures had been opened against 20 of the EU's 27 member states over air quality.
Two countries - Slovenia and Sweden - have been referred to the European Court of Justice over their PM10 levels.
Big fines can be levied if countries are found to have breached EU law.
The Commission says PM10, emitted mainly by industry, traffic and domestic heating, can cause asthma, cardiovascular problems, lung cancer and premature death.
June 2011 is the final deadline before the Commission escalates its legal action against the UK, Mr Potocnik said.
The infringement actions follow the entry into force of a new EU Air Quality Directive in June 2008.
Mr Potocnik said it was not the Commission's job to instruct member states on the methods used to implement the directive.
"States can choose whatever measures they want - they have their own menus. Our concern is the level that should be met," he said.
"We work in good faith with governments. We can't look at all the monitoring stations, check every day," he said.
Last year a scientific study commissioned by London Mayor Boris Johnson estimated that air pollution was responsible for 4,267 early deaths in the UK capital, through long-term exposure.
It found that central London had the worst air pollution in the capital, but outer London had the most early deaths linked to the pollution, due to its higher population.
About 700 people have been arrested throughout Egypt in a crackdown against anti-government protests, security officials say.
The arrests came as police clashed with protesters in two cities following Tuesday's unprecedented protests.
One protester and one policeman were killed as police broke up rallies in Cairo, and in Suez a government building was reportedly set on fire.
Public gatherings would no longer be tolerated, the interior ministry said.
Anyone taking to the streets against the government would be prosecuted, it added.
The BBC's John Leyne in Cairo says the authorities are responding in familiar fashion, treating a political crisis as a security threat.
Prime Minister Ahmed Nazif was quoted as saying the government was committed to "freedom of expression by legitimate means", state news agency Mena reported. Police had acted with restraint, he said.
However, Washington has called on the Egyptian government to lift its ban on demonstrations.
I want to see an end to this dictatorship, 30 years of Mubarak is enough - we've had enough of the state of emergency, prices are going up and up”
End QuoteMostapha al-Shafey Protester
Protesters have been inspired by the recent uprising in Tunisia, vowing to stay on the streets until the government falls.
They have been using social networking sites to call for fresh demonstrations, but both Facebook and microblogging site Twitter appear to have been periodically blocked inside Egypt.
The government denied it was blocking the sites.
Cabinet spokesman Magdy Rady said it respected freedom of expression and "would not resort to such methods", Reuters news agency reported.
In other developments:
Egyptian Trade Minister Rachid Mohamed Rachid cancels his visit to the World Economic Forum in Davos, Switzerland
Activists have called on protesters to observe "Anger Friday", by going to rallies after praying in mosques and churches
In the northern city of Machala, police have cordoned off the headquarters of the Democratic Front opposition party, which is threatening a hunger strike
Hundreds have been arrested in Alexandria, activists say, as police prevent organised rallies
Stone-throwing
Following a "day of revolt" across Egypt on Tuesday, in which four people died, protesters attempted to stage new demonstrations in Cairo on Wednesday.
Police moved to break up demonstrations as they happened
There were scuffles outside the journalists' union building in central Cairo as hundreds of people gathered to protest.
Police beat some with batons and fired tear gas when they tried to break through a cordon.
Protesters burned tyres and threw stones at police.
Reuters news agency reported more clashes outside a central court complex in the city.
Witnesses said riot police had been charging demonstrators throughout the day wherever in Cairo they happened to gather.
Doctors said a policeman and a protester were killed in the clashes, apparently during stone-throwing in a poor neighbourhood of the city.
However, security officials said the deaths were unrelated to the protests.
Meanwhile, in the eastern city of Suez, protesters threw petrol bombs at a government building, setting parts of it on fire, witnesses said.
The headquarters of the ruling National Democratic Party in the city was also attacked.
Earlier, protests were held outside the morgue where the body of a victim of Tuesday's protests was being kept. At least 55 people were injured in the city.
One of Tuesday's demonstrators, Mostapha al-Shafey, told the BBC he planned to join protests again on Wednesday.
"I want to see an end to this dictatorship. Thirty years of Mubarak is enough. We've had enough of the state of emergency. Prices are going up and up," he said.
Demonstrations are illegal in Egypt, which has been ruled by President Mubarak since 1981. The government tolerates little dissent and opposition demonstrations are routinely outlawed.
Social media's role
Tuesday's protests were co-ordinated through a Facebook page, where organisers say they are taking a stand against torture, poverty, corruption and unemployment. One page called for protesters all over Egypt to gather after prayers on Friday.
However, Wednesday brought reports that Facebook was being blocked inside Egypt.
Twitter also played a key part, with supporters inside and outside Egypt using the search term #jan25 to post news on Tuesday, but it was blocked later in the day.
BBC technology correspondent Mark Gregory said that while this clampdown had undoubtedly restricted access to information, technically minded protesters had found ways of evading the restrictions.
Many have stayed in touch by routing their messages through proxy servers - web facilities based in other countries.
The government blamed the violence on the banned Islamist movement the Muslim Brotherhood, although this group was reported to have been ambivalent about the protests.
One opposition leader, Mohamed ElBaradei, had called on Egyptians to take part in the protests.
US Secretary of State Hillary Clinton said: "We urge the Egyptian authorities not to prevent peaceful protests or block communications including on social media sites.
"We believe strongly that the Egyptian government has an important opportunity at this moment in time to implement political, economic and social reforms to respond to the legitimate needs and interests of the Egyptian people."
Tunisia's President Zine al-Abidine Ben Ali was ousted from power and fled the country earlier this month, after weeks of protests in which dozens of people were killed.
Egypt has many of the same social and political problems that brought about the unrest in Tunisia - rising food prices, high unemployment and anger at official corruption.
However, the population of Egypt has a much lower level of education than Tunisia. Illiteracy is high and internet penetration is low.
There are deep frustrations in Egyptian society, our Cairo correspondent says, adding that Egypt is widely seen to have lost power, status and prestige in the three decades of President Mubarak's rule.
Tunisia has issued an international arrest warrant for ousted President Zine al-Abidine Ben Ali and his family, the nation's justice minister has said.
Lazhar Karoui Chebbi said the interim government had asked members states to work via Interpol to detain Mr Ben Ali, who fled to Saudi Arabia on 14 January.
Mr Chebbi said Mr Ben Ali was accused of illegally acquiring property and assets and transferring funds abroad.
The minister was speaking as anti-government protests continued.
Police fired tear gas at hundreds of demonstrators after they reportedly tried to breach barricades around the prime minister's office in the capital, Tunis.
Some in crowd responded by throwing stones at the police.
They were demanding that members of the interim government who served under Mr Ben Ali - including Prime Minister Mohamed Ghannouchi and the defence, interior, finance and foreign ministers - step down.
The interim unity government... has made some encouraging statements and taken some good steps ”
End QuoteJeffrey FeltmanUS Assistant Secretary of State
"We have only one demand: for the government to fall. They all have to go. Ghannouchi should go first," Bassem El Barouni was quoted as saying by the AFP news agency.
Hamid El Gribi, another protester, said: "We have to clean up the rest of the old government."
The prime minister has said he will quit "in the shortest possible timeframe" and promised to hold elections with six months.
He is reportedly preparing to announce replacements for the five opposition ministers who resigned because of the continued dominance of Mr Ben Ali's Democratic Constitutional Rally (RCD) party.
'This woman'
Earlier, Mr Chebbi, a former barrister who did not serve in the last administration, said he had issued an arrest warrant for Mr Ben Ali, his wife Leila Trabelsi, and other members of their extended family, for "illegal acquisition" of assets and "illicit transfers" of funds abroad.
"We are asking Interpol to find all those who fled, including the president and this woman, for trial in Tunisia," he told reporters. "No-one will be above the law."
Tunisians were angered by the power and wealth of Leila Trabelsi's family
The minister said seven family members were currently in Tunisian custody, and also that the head of the presidential guard and five other personnel would be put on trial for "conspiring against state security and inciting people against each other with weapons" after Mr Ben Ali's departure.
Before Mr Ben Ali was overthrown, many protesters expressed their anger at the power, wealth and influence of his wife's family. "No, no to the Trabelsis who looted the budget," was a popular slogan, while the president's relations were referred to simply as "the Mafia".
Later, Interpol confirmed that its bureau in Tunis had issued a global alert - known as an "Interpol diffusion" - requesting that member countries search for, locate and provisionally arrest Mr Ben Ali and six others. The Tunisian authorities would then formally request extradition, it said.
The BBC's Magdi Abdelhadi, in Tunis, says it is unlikely Saudi Arabia will hand them over but the warrants will restrict their ability to travel to other countries, especially in the West.
Earlier this week, the Canadian authorities refused entry to some of Mr Ben Ali's relatives. In France, prosecutors have opened a preliminary investigation into the former president's holdings, believed to range from apartments to racehorses.
Mr Chebbi also revealed that 11,029 prisoners - about a third of those held in Tunisian jails - had escaped during the unrest of the past 12 days.
Of those, 1,532 prisoners had been returned to their cells, he said. Another 74 prisoners were killed in fires that broke out at several jails.
Mr Chebbi also said 2,460 prisoners had been released. He did not say if they were political prisoners, whom the government has promised to free.
Meanwhile, the country's main trade union, the General Tunisian Workers' Union (UGTT), said up to 100,000 people had taken part in an anti-government protest in the second city of Sfax after it called a strike.
Another strike has been called by the UGTT on Thursday in Sidi Bouzid, the town where the uprising began in mid-December.
Protesters are demanding politicians from the old administration stand down
During a visit to Tunis on Tuesday, US Assistant Secretary of State Jeffrey Feltman urged the interim government to do more to satisfy the demands of the people and offered assistance in preparing for free elections.
"The more that the interim government of Tunisia takes concrete steps to prepare for and implement democratic elections and a democratic system, and the more the interim and future governments are able to answer the grievances that led to this popular movement, the stronger and the warmer its partnership with the United States will be," he said.
"The interim unity government... has made some encouraging statements and have taken some good steps regarding the need for elections, for greater openness, for significant reforms.
"These steps will need to be implemented, expanded upon and added to in the days and months ahead."
His comments came as Libyan leader Muammar Gaddafi told Tunisian TV he feared the uprising was being exploited by "foreign interests".
On Wednesday afternoon, the government eased the overnight curfew imposed under the state of emergency because of "improved security". It will now be in place from 2200 (2100 GMT) to 0400 (0300 GMT).
Tens of thousands of Yemenis have demonstrated in the capital Sanaa, calling on Ali Abdullah Saleh, president for 30 years, to step down.
This comes after mass protests in Egypt and a popular uprising in Tunisia that ousted its long-time leader.
Yemeni opposition members and youth activists gathered in four parts of the city, including Sanaa University, chanting anti-government slogans.
They also called for economic reforms and an end to corruption.
Yemenis complain of mounting poverty among a growing young population and frustration with a lack of political freedoms.
The country has also been plagued by a range of security issues, including a separatist movement in the south and an uprising of Shia Houthi rebels in the north.
There are fears that Yemen is becoming a leading al-Qaeda haven, with the high numbers of unemployed youths seen as potential recruits for Islamist militant groups.
'Time for change'
Protesters gathered in several locations of the city on Thursday morning, chanting that it was "time for change", and referring to the popular uprising in Tunisia that ousted President Zine al-Abidine Ben Ali earlier this month.
The BBC's Frank Gardner says these are the largest demonstrations Yemen has seen for some time
Opposition MP Abdulmalik al-Qasuss, from the al-Islah (Reform) party, echoed the demands of the protesters when he addressed them.
"We gather today to demand the departure of President Saleh and his corrupt government," he was quoted as saying by AFP news agency.
Counter-protests have also been staged by the party of President Saleh, the General People's Congress.
Government supporter Saleh al-Mrani said the dissident protesters were a threat to the country's stability.
"We are against whoever wants to trouble the country's interests. All Yemeni people are against that, and we will prevent any kind of disturbance," he said.
President Saleh, a Western ally, became leader of North Yemen in 1978, and has ruled the Republic of Yemen since the north and south merged in 1990. He was last re-elected in 2006.
Yemenis are angry over parliament's attempts to relax the rules on presidential term limits, sparking opposition concerns that Mr Saleh might try to appoint himself president for life.
Mr Saleh is also accused of wanting to hand power to his eldest son, Ahmed, who heads the elite presidential guard, but he has denied the claims.
"We are a republic. We reject bequeathing [the presidency]," he said in a televised address on Sunday.
The editor-in-chief of the Yemen Post newspaper, Hakim al-Masmari, told the BBC World Service that Yemenis were no longer prepared to put up with widespread poverty.
He said the protests were likely to continue because people felt that "all chances of a dialogue with the ruling party are vanishing".
'Tunisia-inspired'
There have been a series of smaller protests in the lead-up to Thursday's mass demonstrations.
Poorest country in the Middle East with 40% of Yemenis living on less than $2 (£1.25) a day
More than two-thirds of the population under the age of 24
Illiteracy stands at over 50%, unemployment at 35%
Dwindling oil reserves and falling oil revenues; little inward investment
Acute water shortage
Weak central government
On Saturday, hundreds of Sanaa University students held competing protests on campus, with some calling for President Saleh to step down and others for him to remain in office.
Over the weekend, Yemeni authorities arrested a prominent rights activist, Tawakul Karman, accusing her of organising the anti-government protests. Her arrest sparked further protests in Sanaa.
After her release from prison on Monday, she told CNN that there was a revolution taking place in her country inspired by Tunisia's so-called Jasmine Revolution.
Protests in Tunisia ended 23 years of President Ben Ali's rule and ignited unrest elsewhere in the region, including Algeria and Egypt.
Authorities in Egypt are bracing for the possibility of further protests, following two days of unrest that have left at least four people dead.
On Wednesday night, activists remained on the streets of the capital, Cairo, and of Suez, defying official warnings.
The government says the protests are illegal and has launched a crackdown, arresting up to 1,000, reports say.
US Secretary of State Hillary Clinton has called on Egypt to "respond to the legitimate needs" of the people.
The country's stock exchange suspended trading temporarily after a sharp drop within minutes of opening, but its benchmark index has fallen further since re-opening.
The BBC's Jon Leyne in Cairo says that while the protesters are still only a minority of Egyptians, they show no sign of fading away and there is a chance that many more people will join once the working week finishes on Thursday.
The government appears to have no answer to the anger and disappointment being expressed on the streets, our correspondent adds - its only response so far has been to crack down on demonstrators and increase security.
People are disgruntled about everything - about politics, economics, their lives, the state of the country. One could talk about the unemployment, and the poverty, and the corruption. People feel they are being treated with contempt by the government.
But the discontent is also wider than that. Egyptians will tell you that this country needs a dream, a vision. They had a dream under President Nasser, they had a dream under President Sadat, they had a dream under the pharaohs.
In the 30-year rule of President Mubarak, there has been no dream - it's been mundane, it's been about numbers, and even on those numbers, many will say they haven't delivered on simple things like education, sanitation, and so forth. So people are really seeing a government and a country in decline.
The opposition figure, Mohamed ElBaradei - the Nobel Peace Prize-winning former head of the UN's nuclear watchdog - says he is returning to Egypt later on Thursday to take part in the protests.
"I am going back to Cairo and back onto the streets, because, really, there is no choice. You go out there with this massive number of people and you hope things will not turn ugly, but so far, the regime does not seem to have got that message," he said in remarks on US website The Daily Beast.
Many Egyptians would no longer tolerate Mr Mubarak's government even for a transitional period, he said, adding that the suggestion that authoritarian Arab leaders like him were the only bulwark against Islamic extremism was "obviously bogus".
"If we are talking about Egypt, there is a whole rainbow variety of people who are secular, liberal, market-oriented, and if you give them a chance, they will organise to elect a government that is modern and moderate."
'We won't stop'
The unrest began on Tuesday in what anti-government activists called a "day of revolt", inspired by the uprising in Tunisia which ousted President Zine al-Abidine Ben Ali.
On Wednesday, they staged fresh demonstrations in central Cairo, despite official warnings that anyone taking to the streets would be prosecuted.
US state department spokesman PJ Crowley: "Egypt needs to reform"
"We've started and we won't stop," one demonstrator told the AFP news agency.
Protesters burned tyres and threw stones at police and there were scuffles outside the journalists' union building, where hundreds had gathered.
Police beat some protesters with batons and fired tear gas when they tried to break through a cordon.
In addition to the four dead, a policeman and a protester were reported killed in Cairo late on Wednesday, although the authorities said this was not related to the demonstrations.
In the eastern city of Suez, at least 55 people were injured.
Egypt's Muslims and Christians will go out to fight against corruption, unemployment and oppression and absence of freedom”
End QuoteActivist
Hundreds of protesters gathered outside the city's morgue, demanding the release of the body of a protester killed there on Tuesday.
Protesters set fire to parts of a government building in the city and attacked the headquarters of the ruling National Democratic Party.
The protests in Cairo continued late into the early hours of Thursday morning - small groups were reported to be breaking off from the main demonstrations, throwing stones at police who responded with tear gas.
About 700 people have been arrested across the country as the authorities have sought to rein in the unrest.
The Associated Press news agency says at least eight journalists are among those detained, including an AP cameraman and his assistant.
The agency says one of its photographers had his cheekbone fractured by a policeman while taking photos of a protest in Cairo on Wednesday.
US caution
The protest organisers have urged people to come out in force again after attending prayers in mosques and churches on Friday, stressing that the religion of protesters is not relevant.
At least 700 people have been arrested in the government crackdown so far
"Egypt's Muslims and Christians will go out to fight against corruption, unemployment and oppression and absence of freedom," Reuters quoted one activist as saying.
The pro-democracy April 6 Movement said it wanted to "continue what we started on 25 January".
"We will take to the streets to demand the right to life, liberty, dignity and we call on everyone to take to the streets... and to keep going until the demands of the Egyptian people have been met," the group wrote on Facebook.
The Egyptian government, ruled by President Hosni Mubarak since 1981, tolerates little dissent and opposition demonstrations are routinely outlawed.
We support the universal right of the Egyptian people, including the right to freedom of expression, association and assembly”
End QuoteHillary ClintonUS Secretary of State
But on Wednesday, Mrs Clinton said it was facing "an important opportunity to implement political, economic and social reforms that respond to legitimate needs and interests of the Egyptian people".
"We support the universal right of the Egyptian people, including the right to freedom of expression, association and assembly," she told reporters at the state department, calling on all parties to show restraint.
Egypt is a key political and military ally of the US in the Middle East, so Washington has been cautious in its response, but it has urged the authorities "not to prevent peaceful protests nor block communications, including on social media sites".
Sites including Facebook and Twitter have been key tools in organising the protests, but are reported to have been blocked across the country at times.
The BBC's Paul Adams in Washington says the US is clearly watching events on the streets of its most populous Arab ally with growing concern, fearing the potential of violent unrest at the heart of the Arab world.
Washington has been reliant on Mr Mubarak's autocratic regime for decades, but the unrest could now mean the US has to tread very carefully as it seeks to influence events in the region, says our correspondent.
Nobel peace laureate and Egyptian opposition politician Mohamed ElBaradei has arrived in Cairo as anti-government protests continue to spread.
A Bedouin protester was shot dead in the Sinai region on Thursday, bringing this week's death toll to seven.
There were also protests in the cities of Cairo, Suez and Ismailiya.
The governing party says it is willing to listen to public grievances such as unemployment but has cracked down on protests, arresting up to 1,000 people.
Speaking on his arrival in Cairo, Mr ElBaradei said he would join the protests
"I wish we did not have to go out on the streets to press the regime to act," he said, according to Reuters news agency.
The protests are expected to increase on Friday, when the weekend begins in Egypt and millions gather at mosques for prayers.
As he left Vienna, where he now lives, Mr ElBaradei told reporters: "If [people] want me to lead the transition, I will not let them down."
'No other option'
Mr ElBaradei, formerly the head of the UN nuclear watchdog, the IAEA, said the government should not use violence against the demonstrators.
Wednesday: policeman and demonstrator killed in Cairo; authorities say deaths not linked to protests
Thursday: anti-government protester shot in Sinai region, witnesses say
"I continue to call on the regime to understand that they better listen and listen quickly, not use violence and understand that change has to come. There's no other option," Mr ElBaradei added.
Meanwhile, the governing National Democratic Party (NDP) of President Hosni Mubarak says it is open to dialogue. But it has warned protesters to remain peaceful, ahead of major demonstrations expected on Friday.
"They are free to express their demands and we are here to meet their needs," said NDP secretary general Safwat al-Sherif.
"I hope that all preachers at Friday prayers tomorrow are calling people to be peaceful in a clear, ritual way that never plays upon people's feelings to achieve an undesirable target."
US President Barack Obama described the protests as the result of "pent-up frustrations", saying he had frequently pressed Mr Mubarak to enact reforms.
"The government has to be careful about not resorting to violence," he said in a YouTube question-and-answer session. "The people on the streets have to be careful about not resorting to violence."
The US counts Egypt as a key ally in the Middle East.
Unrest spreading
Police and demonstrators have again clashed in Cairo and Suez, and the unrest has now spread to the industrial city of Ismailiya.
People are disgruntled about everything - about politics, economics, their lives, the state of the country. One could talk about the unemployment, and the poverty, and the corruption. People feel they are being treated with contempt by the government.
But the discontent is also wider than that. Egyptians will tell you that this country needs a dream, a vision. They had a dream under President Nasser, they had a dream under President Sadat, they had a dream under the pharaohs.
In the 30-year rule of President Mubarak, there has been no dream - it's been mundane, it's been about numbers, and even on those numbers, many will say they haven't delivered on simple things like education, sanitation, and so forth. So people are really seeing a government and a country in decline.
Thursday's protests in Cairo have been on a much smaller scale than the thousands seen on Tuesday and Wednesday. However, in Suez, police fired rubber-coated bullets, tear gas and water cannon, witnesses said.
In Suez, a fire station was set alight by demonstrators.
In Ismailiya, hundreds of protesters fought with police, who used batons and tear gas to disperse them.
The Egyptian stock exchange suspended trading temporarily after a sharp drop within minutes of opening, but its benchmark index has fallen further since re-opening.
The protest organisers have urged people to come out in force again after attending prayers in mosques and churches on Friday, stressing that the religion of protesters is not relevant.
The Muslim Brotherhood has given its backing to the planned protests. Despite an official ban, it remains Egypt's largest and most organised opposition movement.
Egyptian President Hosni Mubarak, 82, has not been seen in public since the protests began on Tuesday.
The Egyptian government tolerates little dissent and opposition demonstrations are routinely outlawed.
Sites including Facebook and Twitter have been key tools in organising the protests, but are reported to have been blocked across the country at times.
Tunisia's interim government has issued an international arrest warrant for the ousted president and his wife, who fled to Saudi Arabia after a popular uprising 12 days ago.
Zine al-Abidine Ben Ali and Leyla El Traboulsi are accused of abusing power to enrich themselves and of taking the money abroad.
The events of the last week will have profound consequences for the Middle East for years to come. Egypt's role in the region is going to change.
President Hosni Mubarak has been the central pillar of the alliance between Western powers and authoritarian Arab leaders and without him it may not be sustainable.
He has been the only Arab leader the Israelis trusted. Their biggest fear is that without him their cold - but so far resilient - peace with Egypt will be in danger.
The president has been the West's necessary man in the Middle East for 30 years.
That is why Egypt has continued to receive vast amounts of American aid, as well as political support from Britain and other European countries - despite a deplorable human rights record, crooked elections, the suppression of virtually all organised political opposition and rampant corruption.
Those are some of the reasons why tens of thousands of people have taken to the streets.
Succession question
His allies were already planning for what would come next, because he is 82.
But for them the easiest assumption was that he would be able to bequeath the Egyptian system largely intact to a chosen successor. Favoured names were his son Gamal, or the intelligence chief General Omar Suleiman.
The crowds on the streets have almost as much contempt for Gamal Mubarak as they have for his father.
Gen Suleiman has been the second most powerful man in Egypt for years, the main link with the Americans, the Israelis and the Saudis.
In the eyes of the protesters though, he has been tainted by agreeing to become a vital part of President Mubarak's survival plan.
The popular uprising makes it unlikely that the current system will survive President Mubarak.
Optimistic Egyptians say free elections, if they ever happen, would produce a vibrant democracy.
Pessimists say that the removal of the police state would lead to chaos - which would be exploited by Egypt's jihadi groups. These have been suppressed ruthlessly by the Mubarak regime.
The country's only properly organised mass political movement outside the ruling party is the Muslim Brotherhood, and it would do very well in any free election.
Unlike the jihadis, it does not believe it is at war with the West. It is conservative and non-violent. But it is highly critical of Western policy in the Middle East.
Egyptian protesters are holding huge rallies in Cairo and other cities as they step up their efforts to force President Hosni Mubarak from power.
Organisers have been hoping to bring one million people on to the streets of the capital. The demonstration is the biggest since the protests began.
The atmosphere has been festive, with protesters singing and chanting.
State TV said Mr Mubarak was preparing to make a statement. Earlier he held talks with a senior US official.
Frank Wisner, a former ambassador to Egypt sent to Cairo on Monday, advised the president not to seek re-election in September or put forward his son Gamal as a candidate for president, senior US officials said.
The BBC's Kim Ghattas in Washington says the US is waiting for an answer, which could come in his statement.
Protest leaders like Mohamed ElBaradei say Mr Mubarak should step down by Friday at the latest.
"They hope that this will end today or Friday at the latest, and they called the coming Friday 'the Friday of departure', but I hope that President Mubarak will take heed before then and leave the country after 30 years of rule and give the people a chance, and I don't expect that he wants to see more blood," Mr ElBaradei told al-Arabiya TV.
Festive atmosphere
BBC correspondents in Cairo's Tahrir Square say the crowds there have been much bigger than on the previous seven days of protests.
From what I saw I would guess there were at least 200,000 people out in Cairo, but it's only a guess. It could be much more. Regardless, nothing like this has ever been seen before in Egypt. The atmosphere of the demonstrations has changed too. The tension and violence of the weekend has gone.
Today was more like a carnival. With the police gone and the military promising not to fire on protesters, whole families came out. The demonstrations were also much more organised. On every street leading to the square, lines of young men and women blocked the way.
They insisted, politely, that everyone show an ID and be searched for weapons. As I was patted down the young man apologised profusely for the inconvenience.
But if the atmosphere has changed the message has not. A huge banner stretched across the middle of the square read: "The People Demand the Removal of the Regime."
Journalists at the scene estimated that hundreds of thousands of people - men, women and children from a cross-section of Egyptian society - were there, although in the absence of official estimates, there is no way of finding out the exact numbers.
The BBC's Jim Muir says that hours after dusk and despite a theoretical 1500 (1300 GMT) curfew and bitter cold the square is still full of people.
Egypt's powerful army has vowed it will not use force against the protesters.
Many carried placards and banners daubed with anti-Mubarak slogans. Earlier, crowds cheered as an effigy of the president was hung from a set of traffic lights in the square.
But the BBC's John Simpson in the square says the demonstration has not been the critical moment people thought it would be, and it has not reached a tipping point.
There is a danger that the demonstrators could lose the initiative if a march to the presidential palace planned for Friday fails to budge Mr Mubarak, but it is hard to see how he can continue in power, he adds.
Meanwhile, new Vice-President Omar Suleiman said he would hold cross-party talks on constitutional reform.
Mr Mubarak reshuffled his cabinet on Monday to try to head off the protests, replacing the widely despised Interior Minister Habib al-Adly.
Prime Minister Ahmad Shafiq told state TV the new government would ensure bread supplies, tackle security problems and "review our entire political, constitutional and legislative situation, into something more satisfactory and appropriate for us as Egyptian citizens".
"Everything is subject to amendments, without limits," he said.
But analysts say the army's statement has been a major blow for President Mubarak, and appears to have encouraged protesters, who are flocking to central Cairo in their thousands.
The UN human rights chief, Navi Pillay, says 300 people may have been killed across the country since the protests began a week ago. They followed an internet campaign and were partly inspired by the ousting of President Zine al-Abidine Ben Ali in Tunisia last month.
Egypt has since cut off the internet in the country and text messaging services have been disrupted.
In other developments:
The US state department announced it had ordered all non-emergency US embassy and government personnel to leave Egypt
AFP news agency reported that US Ambassador Margaret Scobey had spoken by phone to Mr ElBaradei
Most populous Arab nation, with 84.5 million inhabitants
Authoritarian President Hosni Mubarak has ruled for 30 years
Protests against corruption, lack of democracy, inflation, unemployment
Unrest triggered by overthrow of President Zine al-Abidine Ben Ali of Tunisia
Thousands more were out in the streets in Suez, and the Associated Press news agency reported protests in Mansoura, north of Cairo, and the southern cities of Assiut and Luxor.
With limited bus, train and internal flight services, access to the capital has been restricted.
Unnamed security officials were reported as saying all roads and public transportation to Cairo had been shut down.
Some protesters camped out in Tahrir Square on Monday night, saying they would stay there until Mr Mubarak's 30-year rule ended.
One demonstrator, Tarek Shalabi, told the BBC that groups were camped out in tents or sleeping out in the square, and described the atmosphere as "overwhelming".
"We're here because we want to make a statement. We're not going until Mubarak steps down," he said.
He said a stage had been set up where people could go up and make speeches, read out poetry or sing or chant political slogans.
Mubarak supporters have been holding counter-demonstrations in the Egyptian capital
Meanwhile, crowds of pro-Mubarak demonstrators held counter-protests elsewhere in the capital, raising fears of possible confrontations between the different groups.
'Legitimate'
On Monday, the Egyptian army said it respected the "legitimate rights of the people".
In its statement, carried on Egyptian media, the military said: "To the great people of Egypt, your armed forces, acknowledging the legitimate rights of the people... have not and will not use force against the Egyptian people."
Correspondents say the announcement is absolutely critical because it takes away a huge measure of uncertainty from the mind of any potential demonstrator.
A coalition of political opposition groups - incorporating the Muslim Brotherhood, political parties such as that led by Mr ElBaradei, and other prominent figures - has reportedly met, and told the Egyptian government that it will begin talks on its demands only after Mr Mubarak has stood down.
Concerns have also grown about the economy, as global oil prices on Monday topped $100 (£62) a barrel amid fears over the ongoing unrest.
Know what's contagious? Fear is contagious. On the day more than 1 million Egyptians took to the streets demanding the ouster of President Hosni Mubarak, the monarch governing a desert kingdom right next door saw fit to dismiss his Prime Minister and appoint ... a new government.
This was in Jordan, where the statement issued by the Palace of King Abdullah II made clear the first orders to the newly minted Premier, former general Marouf Bakhit: "to take practical, quick and tangible steps to launch true political reforms, enhance Jordan's democratic drive and ensure safe and decent living for all Jordanians."
And so the fever that began in Tunisia and ravaged mighty Egypt appeared to leap the Suez and blister the sands of the Sinai itself before seizing Amman, the well-scrubbed and courtly capital of the Hashemite Kingdom of Jordan. Where, to be sure, there is ample evidence of the ills driving populist revolts across the Arab crescent. On top of stubborn poverty and rising prices for staples like bread, the much vaunted privatizing of the kingdom's public companies swelled the wallets of insiders without reducing a towering public debt. (See the top 10 autocrats in trouble.)
But there are protests, and there are protests. On the streets of Cairo on Friday, Jan. 28, angry youths beat back riot police and set fire to armored trucks. At the same hour in Amman, "the security forces were handing out water and juice," says Joost Hiltermann, an International Crisis Group official who was visiting from Washington. "It was really tepid and friendly."
Consider: The Muslim Brotherhood, known in Jordan as the Islamic Action Front, is calling not for regime change but for "dialogue." It presented its priorities on Monday, Jan. 31; prominent among them were election of the Prime Minister, who, under the terms of the 1952 constitution, is appointed by His Highness. "Today everybody agrees that they do not want regime change. They want reforms," Zaki Bani Rashid told Agence France-Presse (AFP). "But our demands today could change tomorrow if the authorities do not act quickly."
That was Monday. The Prime Minister went the next day.
There was precedent for the housecleaning as well as fortuitous timing. "This happens frequently in Jordan [that the King dismisses the government]," says Michael Young, opinion-pages editor of Lebanon's Daily Star. "The government was perceived to be a corrupt government. It was made up of businessmen who profited from their positions. So the King probably, considering what is going on elsewhere, was trying to block similar events in Jordan. It was a pre-emptive move."(See pictures of mass demonstrations in Egypt.)
"There is no comparison between Egypt and Jordan," an Islamic Action Front official told AFP. "The people there demand a regime change, but here we ask for political reforms and an elected government. We recognize and acknowledge the legitimacy of the Hashemites."
The law gives them little choice, of course. Direct criticism of the King is forbidden in Jordan, so the protesters who assembled outside the office of then Prime Minister Samir Rifai on Saturday did well to make their chants specific. "It is time for change. Our problem is Rifai," they chorused.
Independent observers judge that the respect for Jordan's royals is grounded in realpolitik. The Hashemite royal family hails from the Hejaz of what is now eastern Saudi Arabia and was given control of Jordan by the British mapmakers who (with the French) drew the borders of the modern Middle East. Once a collection of nomadic tribes in what was dubbed TransJordan, the modern kingdom is majority Palestinian, having absorbed a huge number of refugees from the land Jewish armies took over in 1948 to create Israel.
The current monarch's charismatic father, King Hussein, not only navigated the demographic landscape but proved so adept at accommodating assorted parties — including Washington — that he made himself seem indispensable. Abdullah II, who ascended to the throne in 1999, has garnered less glowing reviews. But he married a Palestinian, Queen Rania, and when protesters began gathering in Cairo, he had the wits to get out of the castle that is Amman and be seen inquiring as to the welfare of his subjects in the desert south of the capital. (See why Egypt's Hosni Mubarak may be losing support.)
"If such visits had been undertaken" by Cabinet members, the Jordan Times tut-tutted in an editorial this week, "if the grievances of the population had been made known, it could be that we would not have seen demonstrations, like those that occurred of late, against high prices and the low standard of living of some Jordanians."
Time will tell whether changing Prime Ministers does the trick. Before he left, Rifai lowered the price of bread and raised the pay of civil servants. But of the Arab states that may be vulnerable to the fever for greater democracy, Hiltermann places the Hashemite Kingdom well down the list.
"While the sort of demands being voiced across the region are very similar — better governments, accountability, fighting corruption, political reform generally — the countries we're dealing with are very different," says Hiltermann of the International Crisis Group. "The way these demands are being processed is going to differ from country to country. In Jordan we have a situation where a monarchy has been accepted historically as an outsider bringing together different tribes and populations across Jordan, especially the Palestinians. The monarchy plays a very specific role and has been accepted because of it. I think in Jordan the Hashemite monarchy has a lot of good will."
Young of the Daily Star agrees. "Jordan is different from the region," he says. "There are problems. But the monarchy is not the same as a Mubarak. Mubarak may have behaved like a King, but he was a tyrant, and perceptions are different. No one is going to demand the removal of the Hashemite dynasty. Sure, they can demand that the government be changed, or the removal of parliament, but things in Jordan are a little more complicated than they are in Egypt."
With only about 50m left to drill, time is running out for the Russian scientists hoping to drill into Vostok - the world's most enigmatic lake.
Vostok is a sub-glacial lake in Antarctica, hidden some 4,000m (13,000ft) beneath the ice sheet.
With the Antarctic summer almost over, temperatures will soon begin to plummet; they can go as low as -80C.
Scientists will leave the remote base on 6 February, when conditions are still mild enough for a plane to land.
The team has been drilling non-stop for weeks.
"It's like working on an alien planet where no one has been before," Valery Lukin, the deputy head of Russia's Arctic and Antarctic Research Institute (AARI) in St Petersburg, which oversees the project, told BBC News.
"We don't know what awaits us down there," he said, adding that personnel at the station have been working shifts, drilling 24 hours a day.
Microorganisms have already been found in the lake's frozen water
But some experts remain concerned that probing the lake's water - thought by some to be isolated from everything else on Earth - could contaminate the pristine ecosystem and cause irreversible damage.
The sub-glacial lake is located underneath the remote Vostok station in Antarctica.
Overlaid by nearly 4km of ice, it has been isolated from the rest of the world for millions of years. Some scientists think the ice cap above and at the edges has created a hydrostatic seal with the surface, preventing lake water from escaping or anything else from getting inside.
And if the Russian team gets through to the pristine waters, they hope to encounter life forms that have never been seen.
Astonishing discovery
It was at the Vostok station that the coldest temperature ever found on Earth (-89°C) was recorded on 21 July 1983.
Normally, water in such extreme conditions exists only in one state: ice. And when, in the 1970s British scientists in Antarctica received strange radar readings at the site, the presence of a liquid, freshwater lake below the ice did not instantly spring to mind.
After three km and as we near the bottom [of the ice sheet], the ice temperature gets very close to the ice melting point, and all sorts of problems begin”
End QuoteAlexey EkaikinVostok station, Antarctica
It was not until 1996 that the discovery was formally acknowledged, after satellites sent in the images outlining the lake's contours.
Space radar revealed that the sub-glacial body of fresh water was one of the largest lakes in the world - and one of some 150 subglacial lakes in Antarctica.
At 10,000 square km and with depths reaching 800m, it is similar to Lake Baikal in Siberia or Lake Ontario in North America.
Since the lake has remained sealed off from the rest of the world, scientists estimate that conditions in it have probably remained unchanged for some 15 million years.
For liquid water to exist in Antarctica, glaciologists suggest that the ice cap serves as a giant insulating blanket, able to capture the Earth's geothermal heat to melt the bottom of the ice sheet.
Eager to explore the ancient lake, scientists started drilling and managed to go as deep as about 3,600m - but when the untouched waters were only some 130m away, in 1998, the project ground to a halt.
Antarctica's Vostok station was built in 1956
"We had to stop because of the concerns of possible contamination of the lake," explained Alexey Ekaikin, a member of the current expedition, who spoke to the BBC Russian Service from Vostok station.
He said that drilling was resumed in 2004, when the team came up with new, ecologically safe methods of probing the lake.
We have to make a huge effort not to spoil the environment by being interested in it”
End QuoteDr Andy SmithBritish Antarctic Survey
In November 2010, the scientists submitted a final environmental evaluation of the project to the Antarctic Treaty's environmental protection committee and were given the go-ahead to sample the ancient waters.
They said that instead of drilling into the lake, they would go down until a sensor on the drill detects free water.
Then they would take the drill out without going any further and adjust the pressure so that instead of any liquid in the borehole falling down into the lake, water in the lake would be sucked up.
Then the drill would be taken away and left for quite some time to freeze, creating a plug of frozen ice in the bottom of the hole.
Finally, next season, the team would drill down again to take a sample of that ice and analyse it.
But the work has not been going very smoothly, being repeatedly delayed because of technical difficulties.
In the summer season (Dec to Feb), air temperature is relatively mild, about -30C, but in the winter it can plummet to -80C
"Up until three km down, drilling is usually relatively easy - it has been done in Greenland and here in Antarctica. But after three km and as we near the bottom [of the ice sheet], the ice temperature gets very close to the ice melting point, and all sorts of problems begin," said Dr Ekaikin.
Dr Lukin added that additional difficulties arise from the changing structure of the ice - after about 3,600m, it is pure frozen lake water, composed of huge round monocrystals of a metre or more in diameter and as hard as glass.
That is why for the past few weeks, the team had been advancing at a snail's pace - about 1.6m a day.
They have already reached the 3,700m mark and have just some 50m more to go.
Dr Ekaikin said that having analysed the ice cores obtained so far, the scientists have already discovered some bacteria that are likely to be living at the bottom of the lake, where the water is warmer because of the heat coming from the Earth.
Contamination concerns
Besides possibly discovering new microorganisms, sampling the waters could also move us a step closer to the understanding of similar glacial conditions at one of Jupiter's moons, Europa.
Its surface, researchers suspect, is covered by a huge ocean, hidden within a thick shell of ice.
The ice is extremely hard, which has caused the team problems while drilling
Despite all the precautions, some international observers still dub the project a threat to the ancient sub-glacial lake.
"It's probably almost impossible to make something absolutely, utterly and totally clean," said Dr Andy Smith, a glaciologist at the British Antarctic Survey.
"It's worth [sampling the waters], as even though originally it seemed a really wild thing to expect, there will be life there - anywhere we go on the planet where there's an extreme environment, we always find life.
"But we have to make a huge effort not to spoil the environment by being interested in it," he added.
But the Russians working in Antarctica believe that the risks are virtually non existent and that the possibility of a great discovery makes it entirely worthwhile.
In 2006, researchers reported evidence for a network of rivers under the ice which connect Antarctica's sub-glacial lakes. Some scientists think this could spell trouble for the prospects of finding microbial life that has evolved "independently".
Nevertheless, some of those on the team working at Lake Vostok have been waiting for a eureka moment for decades, and have been coming to the base to drill since the discovery of the lake in the 1970s.
Now they are hoping the technology will not fail them and they will be able to reach the waters before the season ends on 6 February.
The scientists say that at the base, there are only two colours: white and blue; but the scenery is still breathtaking
Because if not, they will have to stay patient for yet another long year.
