By DAVID D. KIRKPATRICK

Egyptian protesters prayed Saturday in front of a military vehicles in Tahrir Square in downtown Cairo on Saturday More Photos

CAIRO – President Hosni Mubarak struggled to maintain a tenuous hold on power on Saturday as the military did not act against tens of thousands of demonstrators who defied a new curfew to call for an end to his nearly 30 years of authoritarian rule.

As street battles between protesters and the security police flared for a fifth day, Mr. Mubarak named a new government filled with military figures, signaling the pivotal role the military might play in shaping the outcome of the unrest. He appointed Omar Suleiman, his right-hand man and the country’s intelligence chief, as vice president and a retired general as prime minister.

Until now, Mr. Mubarak, who was vice president when he took power after the assassination of President Anwar el-Sadat, has steadfastly refused to name any successor, and Mr. Suleiman’s appointment stirred speculation that the president was planning to resign. The new prime minister is Ahmed Shafik, a retired air force general.

Late Saturday, it was still unclear whether the military was defying orders to crack down or simply had not been issued them yet. But at least some troops seemed to be sympathizing with the protesters. In the most striking instance, four armored military vehicles moved at the front of a crowd of thousands of protesters in a pitched battle against Egyptian security police officers defending the Interior Ministry.

Protesters there crouched behind armored trucks as they advanced on the police line surrounding the building, then darted forward to hurl rocks or Molotov cocktails and to set abandoned cars on fire.

But the soldiers refused protesters’ pleas to open fire on the security police. And the police battered the protesters with tear gas, buckshot and rubber bullets. There were pools of blood in the streets, and protesters carried at least a dozen wounded from the front line of fighting.

Everywhere in Cairo, soldiers and protesters hugged or snapped pictures together on top of tanks. With the evident consent of the soldiers, protesters had scrawled graffiti denouncing Mr. Mubarak on many of the tanks in downtown Cairo. “This is the revolution of all the people,” read a common slogan. “No, No, Mubarak” was another. In Alexandria, demonstrators took tea to troops.

The loyalty of the military – the country’s most popular and respected institution – will be crucial to determining whether Mr. Mubarak can remain as the president of his country, a leader in the Arab world and perhaps America and Israel’s closest ally in the region. A change in leadership here would threaten to upend the established order throughout the Middle East.

The unrest continued to reverberate throughout the region, where other autocratic leaders have long held on to power.

King Abdullah of Saudi Arabia blamed unnamed agitators for the demonstrations in Egypt. The Saudi Press Agency quoted him saying: “No Arab or Muslim can tolerate any meddling in the security and stability of Arab and Muslim Egypt by those who infiltrated the people in the name of freedom of expression, exploiting it to inject their destructive hatred.”

And in Yemen, dozens of protesters took to the streets of Sana in solidarity with Egyptian demonstrators, local media reported. There were large antigovernment demonstrations in Yemen last week, as government critics were inspired after street demonstrations toppled Tunisia’s government.

It was unclear what type of jockeying for leadership was going on behind the scenes in Cairo Saturday. But in an interview with Al Jazeera, Mohamed ElBaradei, the Nobel laureate and opposition leader, demanded that the president step down immediately and said the country should “head toward a national unity government.” He did not say specifically what groups might be included or whether the ruling party should participate, but said it should “represent all components of the Egyptian people."

The late-afternoon confrontation at the Interior Ministry in Cairo followed a night of rampant looting around Cairo and then an extraordinary day of peaceful celebration in central squares of the city. The brigades of security police officers who battled hundreds of thousands of protesters on Friday had withdrawn from most of the city, many pulling back to positions defending core government buildings and Mr. Mubarak’s presidential palace.

One crowd cheered and chanted, “The army and the people will purify the country.” And jubilant crowds marched with their fists in the air, many of them carrying Egyptian flags.

By midday Saturday, young civilians were trying to fill gaps left by the police, directing traffic and in some cases defending their neighborhoods with clubs and other makeshift weapons.

Mr. Mubarak, however, appeared to push back, imposing a new curfew of 4 p.m. – which protesters defied – and state television warned that the police would shoot violators on sight.

While some Egyptians reveled in what appeared to be their new freedom, there were ominous signs of lawlessness in places where the police had abandoned their posts.

In the northern port city of Alexandria, witnesses were unnerved by the young men on patrol with sticks, clubs and other weapons.

“We’re Egyptians. We’re real men,” said a shopkeeper, brandishing a machete. “We can protect ourselves.”

Peter Bouckaert, emergencies director of Human Rights Watch, said that he observed a group of soldiers completely surrounded by people asking for help in protecting their neighborhoods. The army told them they would have to take care of their own neighborhoods and that there might be reinforcements Sunday.

“Egypt has been a police state for 30 years. For the police to suddenly disappear from the streets is a shocking experience,” Mr. Bouckaert said. “Even though the police were very repressive, they were also ever-present.”

Although cellphone service was restored in much of the country, the government appeared to still be blocking or restricting the Internet in an attempt to keep protesters from using social networking sites to communicate. The leaders of the early demonstrators, many of them young, used those sites to organize their protests, successfully evading Mr. Mubarak’s efficient security apparatus, which has for years co-opted opposition leaders it could and jailed those it could not.

State television also announced Saturday night that the Egyptian authorities had arrested an unspecified number of members of the Muslim Brotherhood, the Islamist group that for decades has been considered the only substantial counterweight to Mr. Mubarak’s power. When young online activists declared a day of protest on Jan. 25, the Brotherhood initially demurred. But when the size of that outpouring became clear, the Brotherhood said it would turn its members out in full force Friday.

On Friday and Saturday there were many signs of Brotherhood members marching and chanting in the crowds. But the throngs – mostly spontaneous – were so large that the Brotherhood’s presence seemed far from dominant. And questions about the Brotherhood amid protesters often produced passionate debates for and against the group and its potential future influence.

Before the street fights late Saturday, government officials had acknowledged more than 40 deaths around Cairo, plus 27 in Alexandria, 12 in Suez and more fatalities in a handful in other cities. Officials said that as many as 1,000 had been injured. But the final death toll was expected to be much higher. One doctor in a crowd of protesters said his Cairo hospital alone had seen 23 people dead from bullet wounds, and he showed digital photographs of the victims.

Many of the protesters took American journalists aside to complain about United States government support for Mr. Mubarak or to express disappointment with President Obama. Perhaps because of Mr. Obama’s Muslim family history or perhaps because of his much-publicized speech here at the start of his presidency, many of the protesters expressed their criticism in the same unusually personal way: “I want to send a message to Obama.”

In Alexandria, where some of bloodiest clashes with the police took place, protesters’ positions appeared to be hardening.

“I’ve been in the streets from the 25th on, and I’m going to remain in the streets until Hosni Mubarak and his friends leave the country,” said Marwat Saleh, 43, who owns a small tourism company.

“It would have been better if he had not given his speech yesterday, because it seems he did not understand our demands,” she said. “We want him to step down, not only the government; he has to go.”

Mr. Mubarak’s speech just after midnight, in which he dismissed his cabinet, was mainly a defense of his government and the imperative to maintain stability.

Protesters in the city also voiced significant anger at the United States, rushing up to American reporters on the streets unprompted to ask why the United States continued to back the Egyptian government.

“We are very disillusioned by President Obama’s speech,” said Muhammad Shafai, 35, a lawyer, who called for Mr. Obama to distance himself from Mr. Mubarak.

In his speech Friday night, Mr. Obama took on a stern tone, saying he had personally told Mr. Mubarak that he needed to listen to his people’s demands for a “better democracy.” But the United States has counted on Egypt for help in the region, whether supporting American moves in Iraq or trying to defuse tensions between the Palestinians and Israelis.

In Sinai, officials said that the security police had withdrawn from broad portions of the territory, leaving armed Bedouin in control. At least five members of the police, both law enforcement and state security, were killed, officials said.

The army moved to secure Cairo International Airport on Saturday as The Associated Press reported that as many as 2,000 people had flocked there in a frantic attempt to leave the country. It was unclear how quickly they could leave, however, since international carriers reported delays and cancellations.

The Israeli government, meanwhile, flew the families of its envoys, as well as 40 other Israeli citizens, out of the country on a special flight.

But by Saturday night, much of Cairo – including the upscale Zamalek area – was in the control of young civilians armed with clubs and bats. They said they were armed to deter looters and protect their neighborhoods, and they stopped cars and detained passers-by, ostensibly for breaking the curfew. Gunfire was heard in Cairo and some suburbs.

Reporting was contributed by Scott Nelson, Kareem Fahim and Mona El-Naggar from Cairo, Souad Mekhennet and Nicholas Kulish in Alexandria, and J. David Goodman and Ali Adeeb from New York.