Leading Egyptian opposition figure Mohamed ElBaradei has joined thousands of protesters in Cairo defying a curfew to demand President Mubarak stand down.
Addressing the crowd in Tahrir (Liberation) Square, the Nobel peace prize winner urged patience, saying "change is coming".
In a tense sixth day of protests, the army sent tanks into the square only to see them blocked by demonstrators.
Air force jets and a helicopter repeatedly flew low over the square.
"You have taken back your rights and what we have begun cannot go back," Mr ElBaradei told the crowd.
"I bow to the people of Egypt in respect. I ask of you patience, change is coming in the next few days."
Mr ElBaradei has been asked by opposition groups to negotiate with the authorities on forming a national unity government.
However, the BBC's Jim Muir in Cairo said the former UN official did not attract the attention of the whole crowd, which contained other political factions.
Embattled President Hosni Mubarak earlier met top commanders during a visit to a military headquarters.
Over the weekend, US President Barack Obama made a number of calls to foreign leaders to discuss the situation in Egypt, the White House said.
A statement said Mr Obama had spoken to Turkish Prime Minister Recep Tayyip Erdogan, Prime Minister Benjamin Netanyahu of Israel, King Abdullah of Saudi Arabia and British Prime Minister David Cameron. He asked them for their assessment of the situation and agreed to stay in close contact, it said.
Mr Cameron's office said he and Mr Obama were "united in their view that Egypt now needed a comprehensive process of political reform".
Echoing an earlier US statement, a spokesman for Mr Cameron said the two leaders wanted to see an "orderly, Egyptian-led transition" leading to a democratic future for Egypt.
Mr ElBaradei, however, has said Washington is losing credibility by talking of democracy while still supporting a president viewed by Egyptians as oppressive.
The BBC's Jeremy Bowen, in Tahrir Square, said the arrival of a column of tanks and the sound of jets roaring overhead significantly raised the tension among the crowds.
In Cairo on Sunday, there was no sign of the riot police with whom protesters have clashed violently in recent days, although state television said they were being ordered back on to the streets.
Late Sunday night, state television also reported that the 1600 (1400 GMT) curfew - which has been widely ignored - would be extended in Cairo, Alexandria and Suez to run from 1500 to 0800.
Clashes are reported to have left at least 100 people dead since rallies began on Tuesday in cities including Cairo, Suez and Alexandria. Thousands more have been injured.
Although Sunday is the start of the working week in the Middle East, many businesses in Cairo remained closed and internet access is still intermittent.
In Alexandria, in the north, the BBC's John Simpson said more protests were being held there and the army was out in force.
In the capital, citizens have formed armed groups to protect their property from looters.
Across the country, thousands of prisoners are reported to have escaped from jails after overpowering their guards.
They are said to include senior members of the Muslim Brotherhood - the largest opposition group in Egypt.
On Sunday, a number of Egyptian political movements issued a joint statement calling on Mr ElBaradei to form a transitional government. The Muslim Brotherhood is also reported to have asked him to negotiate with Mr Mubarak's regime.
Mr ElBaradei, a former head of the UN's nuclear watchdog, is a leading advocate for political reform and a likely challenger for Egypt's presidency.
Correspondents say that although he has become a leading figurehead of the opposition, protesters have not yet called for him to be their leader.
President Mubarak, in an attempt to regain control, has appointed a vice-president - intelligence chief Omar Suleiman. Aviation Minister Ahmed Shafiq has been appointed prime minister.
The US embassy in Cairo said on Sunday it would begin flying Americans out of Egypt on Monday.
It had earlier advised US citizens in Egypt to consider leaving as soon as possible.
The UK is advising its nationals in Cairo, Alexandria and Suez to leave if it is safe for them to do so.
A number of other European countries have also advised against visiting the country.
The unrest in Egypt follows the uprising in Tunisia which ousted President Zine al-Abidine Ben Ali two weeks ago after 23 years in power.
The Tunisian turmoil began with anger over rising food prices, high unemployment and anger at official corruption - problems which have also angered many people in Egypt.