Tens of thousands of demonstrators remain on the streets of Egypt's major cities, despite a curfew which came into force at 1600 local time (1400 GMT).
The army has warned that anyone who breaks the curfew will be in danger.
In Cairo, police have used rubber bullets and tear gas in clashes with protesters, but the army has yet to step in. Injuries are reported.
There have also been clashes in the cities of Alexandria and Ismailiya.
Cairo's central Tahrir (Liberation) Square remains filled with protesters. Troops and armoured vehicles have been deployed but have not yet taken any action.
A BBC Arabic correspondent at the scene reports a friendly atmosphere between the army and the demonstrators.
Health officials say 45 people have died in clashes across Egypt since Friday.
The latest figures bring the death toll in the week's unrest to at least 52, with both protesters and police officers among the dead. About 2,000 people have been injured.Turmoil
A curfew in Cairo, Alexandria and Suez has been extended to last between the hours of 1600 and 0800 (1400 GMT and 0600 GMT).
The army has advised people to obey the curfew and avoid gathering in groups.
Hundreds of foreign tourists and Egyptian nationals are at Cairo's main airport seeking flights out of the country.
The Egyptian military has used tanks and armoured personnel carriers to seal off the site of the pyramids on the Giza Plateau, the Associated Press reports.
The army had already secured the Egyptian Museum, home to such treasures as the gold mask of King Tutankhamen, to protect it from looters.
In Abu Za'abal prison in Cairo, a political prisoner has told the BBC that 120 inmates have taken control of one sector of the jail.
Speaking by mobile phone, Mohamed Mahmud Sami - who has been in prison for 17 years - said: "Security forces are trying to storm in, but we can see that the soldiers are reluctant to fire at us, as if they want to side with the rebelling people of Egypt."
Cairo stock exchange will be closed on Sunday - a full trading day in the Middle East - because of the turmoil in the city.
On Friday, tens of thousands of protesters defied a curfew and remained on the streets into the night. In Cairo, they set fire to the headquarters of the governing NDP party and surrounded state TV and the foreign ministry.
Mr Mubarak responded by sacking his government. In a televised address - his first public comments since the protests began on Tuesday - he also accused the protesters of destabilising the country.
The government has now formally resigned, with reports suggesting that Racheed Mohamad Racheed - current minister of investment, commerce, and industry - will be named as the new prime minister.Defiance
Mobile phone services have been restored in Cairo, but the internet remains down.
The BBC's Yolande Knell, in the Egyptian capital, says there is anger that demands for political reform are not being listened to. Protesters have said they will ignore the curfew and stay on the streets until there is change, she adds.
In Suez, soldiers are on the streets after the city's police and authorities fled following Friday's violence in which the main police station was burned down.
Reports from the northern coastal city of Alexandria and the eastern city of Ismailiya say thousands of protesters are on the streets, with clashes occurring between polices and demonstrators.
And a BBC Arabic producer reports that inmates have rioted in the city of al-Manufiya, north-west of Cairo.
US President Barack Obama, who telephoned President Mubarak on Friday, has said a violent response by the Egyptian authorities will not address the grievances of the people. He urged protesters to desist from violence, and also called on the Egyptian government to stop interfering with the internet, social networking sites and mobile phone services.
White House spokesman Robert Gibbs said Washington would review its aid to Egypt based on events in the coming days. Egypt is the fourth largest recipient of US aid, after Afghanistan, Pakistan and Israel.
Britain, the US and France are all advising against non-essential travel to Egypt.
The unrest in Egypt follows an uprising in Tunisia two weeks ago which toppled President Zine al-Abidine Ben Ali after 23 years in power.
The Tunisian upheaval began with anger over rising food prices, high unemployment and anger at official corruption - problems which have also left many people in Egypt feeling frustrated and resentful of their leadership.