By CHARLES LEVINSON, MARGARET COKER and SUMMER SAID

CAIRO—Egypt’s opposition groups lined up behind a moderate leader comfortable on the world stage as their best chance to oust President Hosni Mubarak Sunday, while the nation’s military closed ranks with the government leadership but allowed protests to continue raging in the streets.

The moves continued to sharpen the country’s clash over whether Mr. Mubarak would resign. And events here present difficult choices for the U.S.—which has been attempting to push for both the stability that the military offers and the sweeping political changes demanded by the opposition. There was no indication that the two sides would meet or hold discussions.

State television showed footage of Mr. Mubarak with his newly appointed vice-president, former intelligence chief Omar Suleiman, meeting Egypt’s top army commanders Sunday. The images appeared to be a bid to show that control of the armed forces was still in the hands of Mr. Mubarak and his regime.

Fragmented opposition groups, including the powerful Muslim Brotherhood, put aside sometimes strident differences to unify behind Mohamed ElBaradei, a Nobel Peace Prize laureate who often tangled with U.S. officials when he led the United Nations agency inspecting Iran’s nuclear program. His entry in Egyptian politics is more recent. He came to Cairo last week only after the protest movement had gathered steam on its own.

He said in televised remarks that he looked forward to working with the military to help establish order and forge Egypt’s new political future.

In an interview with the British Broadcasting Corporation, he said: “The first step is [Mubarak] has to go…The second step is we have to have a government of national salvation in coordination with the army. The third step is the army has that horrible task of ensuring security.”

Mr. ElBaradei’s public acknowledgment of the crucial role the military would play in any transition appeared to be an attempt to win over a key pillar of the regime. Ever since a military-led coup seized power in 1952, the military has wielded considerable power, but in recent decades has kept a relatively low profile in internal politics.

Mr. ElBaradei’s endorsement Sunday by the Muslim Brotherhood, Egypt’s largest and best-organized opposition force, amounted to a historic display of unity between the country’s normally splintered secular and Islamist opposition forces. The development underscores what appears to be a new strategy by the Brotherhood to temper its religious platform in support of what opposition groups argue is a battle for democracy. It also suggests that the group’s once-sidelined moderate wing could be gaining strength.

One hallmark of the uprising against Mr. Mubarak has been its secular tone. Brotherhood members agreed with the umbrella of opposition groups organizing the protests to keep religious slogans out of the demonstrations. The aim was to minimize the risk that Mr. Mubarak’s security agencies could discredit the movement, organizers said.

Egypt’s opposition groups have had a checkered past, with ideological divides and personal animosities sapping them against the might of the Mubarak regime. For now, their solidarity appears to be holding. The umbrella organization on Sunday formed a steering committee under Mr. ElBaradei to pressure the regime for more political concessions, according to senior Brotherhood leaders.

“What we want is what the people want: right now we should have a completely different regime. We should have freedom and free elections,” said Helmi Gazzar, head of the Brotherhood’s district party office in northern Cairo. “We respect Mr. Baradei,” he said. “He has the most potential” to achieve this.

Mr. ElBaradei made his first appearance in the city’s central Tahrir Square on Sunday evening, where tens of thousands of protestors massed. The ministry’s headquarters a few blocks off Tahrir Square was the site a day earlier of violent clashes between protestors and what appeared to be the last vestiges of internal security forces on the country’s streets.

The military appeared to some to come to the protestors’ aid, sending four armored personnel carriers to face down the police lines. Top army officials, including the powerful defense minister, also made appearances around the capital, meeting with troops deployed in the area and shaking hands with people still gathered for anti-government demonstrations.

But Egypt’s massive security establishment consolidated its forces and showed no clear signs of reaching out to the opposition leadership.

The military increased its control over security and political affairs. The army assumed control of the Ministry of the Interior, which oversees the country’s massive internal security forces, loathed by many Egyptians for their brutality.

One of the more dramatic displays of military might came in the late afternoon, when two F-16 jets flew over the Nile and the center of the capital.

A force of roughly 1.4 million, Egypt’s armed forces are the 12th largest in the world, according to the International Institute for Security Studies, and receive $1.3 billion in U.S. aid per year. Military factories control significant chunks of the nation’s economy, giving them a vested interest in controlling the pace and extent of any political reforms that may ensue.

The military has traditionally enjoyed broad respect among Egyptians, and appears to have gone to great lengths to avoid antagonizing protestors and regular citizens since deploying in the streets.

Signs of disorder—including outbreaks of looting and reports of mass prison breaks from at least five different detention facilities since Friday—threaten to erode the military’s standing if it can’t manage to impose order. The army has yet to take firm control of the streets, in the absence of the police, leaving many Egyptians wondering why.

Many opposition leaders and analysts accused the military of allowing a measure of chaos as part of a larger strategy to discredit popular protests. They have also accused the internal security forces of responsibility for much of the looting and chaos. Others believe that the military is simply incapable of restoring order or unwilling to jeopardize its popular standing by taking on the potentially unpopular role of a police force.

Despite the public displays of power, major question marks surround the Mubarak regime. It was unclear how unified the regime’s top leadership was, how deep divisions ran within the military hierarchy, and the extent of cooperation between the military and the police force.

Amid the chaos, state television announced that police forces would return to the streets Monday, but wouldn’t deploy in the city’s central Tahrir Square, apparently to avoid friction with protestors.

—Matt Bradley in Cairo and Adam Entous and Julian Barnes in Washington contributed to this article.Write to Charles Levinson at charles.levinson@wsj.com, Margaret Coker at margaret.coker@wsj.com and Summer Said at summer.said@dowjones.com