CAIRO — Egypt’s military rulers and the Muslim Brotherhood escalated their confrontation on Friday as the generals threatened to use “the utmost firmness” to preserve their authority and the Islamists presented a reunited front with some of their former allies from the revolt against Hosni Mubarak.

More than 100,000 protesters poured into Tahrir Square for a fourth day, demanding that the generals cede power to elected civilians at the end of the month as they had promised. But anxiety was high, and there was talk of a potential explosion after a state-run Web site floated rumors that the election commission would invalidate the results of last weekend’s presidential runoff and declare Ahmed Shafik, a former air force general, the next president. A public vote count showed that the Muslim Brotherhood’s candidate, Mohamed Morsi, had won.

Hopes for a negotiated settlement seemed to fade as both sides began to treat the showdown as a life-or-death struggle. The Brotherhood had already watched the military dissolve the Islamist-led Parliament elected just a few months ago, and the generals now faced calls to block their core demand: a voice in crafting a new constitution that could protect their power and privilege.

“The Armed Forces have emphasized self-control out of respect for the revolutionary state to avoid losses or injuries, as part of the people exercise their right to express their opinions,” a spokesman for the ruling military council declared in a televised statement, suggesting that the military’s patience was nearing an end. “Everyone should respect the principles of legitimacy to avoid the dangers of abandoning them.”

Alluding to the court decision the military used to justify dissolving the Parliament, the military spokesman said: “The verdicts issued by the judiciary are enforced in the name of the people, and refraining from enforcing them or hindering their enforcement is a crime punishable by law.”

Asmaa Waguih/Reuters

Protesters prayed during a demonstration at Tahrir Square in Cairo on Friday.

The generals accused the Brotherhood of causing the political crisis, by “pre-empting the elections results” with the declaration that Mr. Morsi had won.

Brotherhood officials vowed to ramp up their protests until the generals restored the Parliament. And they warned against any attempt to invalidate the vote count in order to make Mr. Shafik the president.

“We expect the high elections committee to announce the result as soon as possible without delay,” Mr. Morsi said. “The expected result is known for everyone. We all won’t allow for anyone to tamper with this result.”

He was flanked by a disparate group of secular and liberal activists, as well as intellectuals not seen together since political division set in after Mr. Mubarak’s ouster last year. Some were more liberal Brotherhood dissidents, like Islam Lotfy, expelled in a bitter dispute; others were secular opponents, like Shady el-Ghazaly Harb, who boycotted the election because they could not vote for an Islamist.

Wael Ghonim, a Google executive who became a hero of the revolt, said the group had come together to hold off a military coup. “Our stand isn’t with the Brothers,” he said. “Our stand is with legitimacy; our stand is with democracy.” He vowed that they would not accept that “democracy will bring those we don’t want to power.”

Panic about the presidency began Friday after a state news media Web site quoted anonymous officials as saying that the Mubarak-appointed election commission would give the presidency to Mr. Shafik. He, too, has confidently pronounced himself the winner.

Brotherhood officials called the reports either a threat or a trial balloon, and other official sources said the election commission was still evaluating charges of fraud. “If they name Shafik, then they have blocked their own tunnel,” said Jihad el-Haddad, a Brotherhood spokesman. “There is no going back, they have tipped the scale.”

A State Department official, speaking on the condition of anonymity under diplomatic protocol, said that in recent days American diplomats had heard alarm about the prospect of a military takeover from their contacts with the Brotherhood. (The United States has cultivated closer ties as the Brotherhood approached power.)

The Brotherhood leaders had previously appeared confident that they could work out an accommodation with the generals about their economic interests or criminal immunity that would allow a formal transition to civilian rule, the official said, and they now seemed very angry at the military’s betrayal.

Secretary of State Hillary Rodham Clinton publicly urged this week that the generals complete their promised exit, and administration officials said others had privately delivered the same message as well. Congress has made the $1.3 billion in annual American aid contingent on steps toward democracy. Some called for a tougher stand. “If we’re not getting any positive response for our money, why should we be spending millions more?” said Senator Patrick Leahy of Vermont, a Democrat with influence over the spending.

At the same time, administration officials worry that America’s regional allies — Israel, Saudi Arabia and the United Arab Emirates — have taken the opposite tack, and are supporting a crackdown in Egypt, said Jonathan Alterman, director of the Middle East program at the Center for Strategic and International Studies. “There’s a deep feeling among a lot of people in Egypt and elsewhere that the U.S. is naïve about the Brotherhood,” he said.

Still, State Department officials said that the administration — all the way up to the president — strongly wanted to support a movement for democracy in Egypt. But the administration found it hard to rally the political will for maximum pressure without a bigger movement in the streets or scenes of official violent repression.

The Brotherhood has pledged to keep the protests peaceful.