The announcement came after the opposition dismissed Mr. Mubarak's cabinet reshuffle as inadequate and as concerns over violence were heightened by the presence of security police officers clustered near the square's entrances, their first deployment there in three days.
Since the demonstrations began last Tuesday, Mr. Mubarak has stayed mostly out of sight, apparently intent on waiting for the protesters' passions to cool. But opposition organizers called for the largest demonstrations yet -- a "march of millions" and a general strike -- on Tuesday, and the Egyptian economy showed more signs of shutting down.
Across the square, trepidation inflected the euphoria. Many protesters suggested that the coming days may be pivotal, as an inchoate movement struggles to maintain the pressure on an entrenched state.
In contrast to previous days in the uprising, which were dominated by the young, the demonstrations Monday included a more obvious contingent of older, disciplined protesters and members of the Muslim Brotherhood. The Islamist organization, outlawed under the Mubarak government, has been playing a steadily larger role in the demonstrations, after holding back at the outset.
The president appeared fatigued in a ceremony broadcast on state television in which he welcomed a new interior minister, Mahmoud Wagdy, a retired general, who will oversee the police. He replaced Habib el-Adly, who had been interior minister since 1997 and came under sharp criticism from human rights advocates for tolerating -- or even encouraging -- torture and other police abuses.
Mr. Mubarak left several longtime associates in place, including the foreign minister, the minister of information and the defense minister.
With the Internet still broadly disrupted, Egyptians gathered at mosques around the city for noon prayers and then marched by the hundreds and thousands toward Liberation (Tahrir) Square on Monday, passing groups of security police and soldiers, .
"I brought my American passport today in case I die today," said Marwan Mossaad, 33, a graduate student of architecture with dual Egyptian-American citizenship. "I want the American people to know that they are supporting one of the most oppressive regimes in the world and Americans are also dying for it."
"Come down, Egyptians!" chanted one group heading to the square, drawing men into their march from the buildings they passed. The group, led by older men, linked hands and kept to one lane of traffic, allowing cars to pass.
At the square, they joined protesters who had stayed all night in defiance of a curfew that the authorities are now seeking to enforce at 5 p.m., an hour earlier. The numbers in the square appeared to exceed those of previous days, despite efforts by the military to corral the protesters into a narrower space.
Army troops checked the identity of people entering the square and began placing a cordon of concrete barriers and razor wire around its access routes, news reports said. But there were no immediate reports of clashes with the protesters, who have cast the military as their ally and protector. As military helicopters circled overhead, demonstrators jabbed their fists in the air, chanting, "The people and the army are one hand."
Witnesses in Alexandria, Egypt's second city on the Mediterranean coast, said police had returned to the streets there, though only in small numbers and accompanied by soldiers.
At Cairo International Airport, a voluntary evacuation of Americans -- including dependents of government officials in Egypt, some diplomats and private citizens -- got under way on Monday with a flight to Cyprus and two to Athens, as passengers waited to board six more flights heading for other unspecified destinations described as safe havens, including Turkey, American Embassy officials said.
International oil companies are closing local offices, evacuating nonessential personnel and family dependents, and telling their Egyptian employees to stay home, but most companies said there had been little impact so far on exploration and production activities centered in the Gulf of Suez, the Western Desert and the Nile Delta.
ENI, the Italian oil company and largest foreign producer in Egypt, said it was operating its production fields normally. BP, the British company with considerable operations in Egypt, also announced that its production had not been affected. Apache Corp., the United States oil company with the greatest exposure in Egypt, said its production operations were normal. After falling on Friday, Apache's stock price recovered on Monday.
The return of police forces to the street came as. Mubarak replaced his brutal interior minister, Mr. el-Adly. Protesters had called for his resignation last week as Egypt's widely reviled security forces cracked down harshly on protesters in Cairo and other cities.
But some analysts dismissed Mr. Mubarak's cabinet changes as window dressing that did not even meet the protest movement halfway.
"It is disappointing," said Gamal Abdel Gawad, of the Ahram Center for Political and Strategic Studies. "There was an opportunity to use this cabinet as the tool to reconcile with the opposition by bringing in political figures from the political spectrum. But now we are back to a government of technocrats. Many of them are very good and well-respected people, but the question now is about politics; it's not about policies."
Israel, meanwhile, granted permission to Egypt to move two battalions of soldiers, about 800, into the Sinai Peninsula, which has been largely demilitarized since the two countries signed a peace treaty in 1979. The area is populated by armed Bedouin tribes that have posed a challenged to the Egyptian authorities over the years.
Jerusalem was also reported to have called on the United States and a number of European countries over the weekend to mute criticism of Mr. Mubarak to preserve stability in the region, the Israeli daily Haaretz reported.
But an Israeli government official, speaking on condition of anonymity following diplomatic protocol, said that the Haaretz report did not reflect the position of the prime minister, Benjamin Netanyahu. Mr. Netanyahu spoke cautiously in his first public remarks on the situation in Egypt, telling his cabinet that the Israeli government's efforts were "designed to continue and maintain stability and security in our region."
"I remind you that the peace between Israel and Egypt has endured for over three decades, and our goal is to ensure that these relations continue," the prime minister said on Sunday as Egypt's powerful Muslim Brotherhood and the secular opposition united around a prominent government critic in hopes of negotiating with the Army for Mr. Mubarak's departure.
The announcement that the critic, Mohamed ElBaradei, would represent a loosely unified opposition reconfigured the struggle between Mr. Mubarak's government and an uprising bent on driving him and his party from power. But it left open the broader question of how to manage the transition to democracy from American-backed governments like Egypt that have proven most adept at eliminating any vestige of it.
Though lacking deep support on his own, Dr. ElBaradei, a Nobel laureate and diplomat, could serve as a consensus figure for a movement that has struggled to articulate a program for a potential transition. It suggested, too, that the opposition was aware of the uprising's image abroad, putting forth a candidate who might be more acceptable to the West than beloved in Egypt.
In a collapse of authority, the police withdrew from major cities on Saturday, giving free rein to gangs that stole and burned cars, looted shops and ransacked a fashionable mall, where dismembered mannequins for conservative Islamic dress were strewn over broken glass and puddles of water. Thousands of inmates poured out of four prisons, including the country's most notorious, Abu Zaabal and Wadi Natroun. Checkpoints run by the military and neighborhood groups, sometimes spaced just a block apart, proliferated across Cairo and other cities.
Many have darkly suggested that the government was behind the collapse of authority as a way to justify a crackdown or discredit protesters' calls for change.
"Egypt challenges anarchy," a government-owned newspaper declared Sunday.
"A Conspiracy by Security to Support the Scenario of Chaos," replied an independent newspaper in a headline that shared space at a downtown kiosk.
Get Breaking news, live coverage, and Latest News from India and around the world on NDTV.com. Catch all the Live TV action on NDTV 24x7 and NDTV India. Like us on Facebook or follow us on Twitter and Instagram for latest news and live news updates.