Mohamed ElBaradei, a leading opposition figure, has joined thousands of protesters in Cairo's Tahrir Square, in continued demonstrations demanding an end to President Hosni Mubarak's 30-year rule.
The former head of the International Atomic Energy Agency told the crowd on Sunday night that "what we have begun cannot go back" referring to days of anti-government protests.
The National Coalition for Change, which groups several opposition movements including the Muslim Brotherhood, wants ElBaradei to negotiate with the Mubarak government.
"The people want the regime to fall," protesters chanted as ElBaradei walked to the centre of the square, holding hands with some demonstrators.
The show of continued defiance by the people came on a day when air force fighter planes flew low over Cairo along with helicopters and extra troop lorries appeared in the central square.
As the protests continue, security is said to be deteriorating and reports have emerged of several prisons across the country being attacked and of fresh protests being staged in cities like Alexandria and Suez.
Thirty-four leaders from the Muslim Brotherhood were freed from the Wadi Natroun jail after guards abandoned their posts.
The protesters in Cairo, joined by hundreds of judges, had gathered again in Tahrir Square in the afternoon to demand the resignation of Hosni Mubarak.
Al Jazeera's correspondent, reporting from the scene, said that demonstrators confronted a fire truck, at which point army troops fired into the air in a bid to disperse them.
He said the protesters did not move back, and a tank commander then ordered the fire truck to leave. When the truck moved away from the square, the thousands of protesters erupted into applause and climbed onto the tank in celebration, hugging soldiers.
Main roads in Cairo have been blocked by military tanks and armoured personnel carriers, and large numbers of army personnel have been seen in other cities as well.
Reporting from Cairo earlier on Sunday, an Al Jazeera correspondent said it was a "long way from business as usual" in the Egyptian capital on the first working day since protests peaked on Friday.
He said that extra military roadblocks had been set up in an apparent attempt to divert traffic away from Tahrir Square, which has become a focal point for demonstrators.
"It's still a very tense scene to have so much military in the capital city of the country."
Earlier in the morning, another correspondent reported that the city appeared deserted in the early hours.
"The streets are very dirty, there is debris everywhere. The police have just disappeared. Any security at this stage is in the hands of the army."
Al Jazeera's correspondents in the port city of Alexandria have also said that anti-government protests have begun there, with hundreds of people on the streets.
The security situation in the capital prompted the country's interior minister to hold meetings with top officials on Sunday.
Habib al Adli met Mohamed Hussein Tantawi, the defence minister, and vice-president Omar Suleiman, state television reported.
As the police withdrew from streets across Egypt, Adli has been the target of growing criticism by the protesters who have called on him to resign.
The absence of police has given looters a free rein, forcing ordinary citizens to set up neighbourhood patrols.
However, police are expected to return to the streets two days after they virtually disappeared, one of Al Jazeera's correspondents in Cairo said.
"We are expecting a statement by the minister of interior on Monday about whether the police are going to return or not.
"They all but disappeared over the last couple of days and many people are wondering where they disappeared to.
"There are two schools of thought as far as the police are concerned: One is that many of them decided to join the protesters. The other is that the regime was saying to the people, 'You want to protest. We'll pull back the police and you feel what anarchy feels like'," our correspondent said.