A showdown between the Muslim Brotherhood and Egypt’s ruling military council escalated Friday, as the generals chided the group for claiming victory in last weekend’s presidential runoff election and the Islamist candidate pledged defiance.

The dueling challenges were delivered as tens of thousands of Egyptians swarmed into Cairo’s Tahrir Square for a Brotherhood-led protest against attempts by the armed forces to consolidate power. Those moves include a recent constitutional decree giving the military vast authority and enfeebling the future president.

As the crowd swelled, the generals issued a statement defending the decree as in the nation’s “interest” and promising swift and firm action against those who fueled unrest.

Egypt’s presidential election commission had been expected to announce the winner Thursday, but the proclamation was postponed. Mohamed Morsi, the Brotherhood’s candidate, has claimed victory and provided precinct tally reports as evidence. His rival, Ahmed Shafiq, who is widely presumed to be the military’s candidate, has also said he won the election but has not supported his claim with evidence.

Concern grew Friday that the competing claims and the military’s seeming reluctance to transfer power to an elected president could set the stage for a violent confrontation.

The week-long delay in announcing a victor also fueled speculation that the military junta might rig the results after striking a deal with one of the candidates.

“They’re negotiating the winner,” said Shadi Hamid, an Egypt expert at the Brookings Doha Center. “There’s no other explanation I can think of for this long of a delay.”

Friday’s military statement called the announcement of unofficial results by the Brotherhood “unjustified” and “one of the main reasons behind the prevailing division and confusion in the political arena.”

Hours after the statement’s release, Morsi appeared at a news conference with a host of Brotherhood stalwarts, public intellectuals and youth leaders viewed as secular. They said they had set aside differences, if only for the integrity of the electoral process, and would unite to oppose the military’s maneuvers.

But the type of political heavyweights who could have made up a formidable front against the generals were conspicuously absent from the stage. Many have long been wary of the Brotherhood, perceiving it as a group that is quick to cut deals and looks solely after its own interests. Liberal political groups are reluctant to back the Brotherhood because they worry about the prospect of a dogmatic government.

“Even facing the rise of the old regime, the left and the Brotherhood have been unable to form a united front,” Hamid said. “That’s what the military is counting on: divide and conquer.”

In a strident but diplomatically worded speech, Morsi appeared to rebuke the military council for its recent moves and for its warnings against peaceful protests.

“The constitutional declaration clearly implies attempts by the military council to restrict the incoming president,” he said, at times drawing applause from the Egyptian press corps. “This we totally reject.”

Morsi said the Brotherhood had been meeting for two days with a broad range of politicians, activists and other public figures who had agreed to join forces in support of a smooth and meaningful transfer of power, which the military has promised by July 1. But he also appeared disinclined to provoke the military, and he ended his statement with conciliatory words for the armed forces.

“They are honest men working for the betterment of Egypt,” Morsi said. “They are working to safeguard our land.”

As he has done in the past, Morsi sought to reassure those skeptical about an Islamist president, saying he would form a broad-based cabinet led by an independent prime minister and including women and Christians.

He did not say whether he or other Brotherhood leaders had been in discussions with the military, but he insisted that there would be no compromise on the demand to overturn the military decree. In an interview Wednesday, Brotherhood spokesman Mahmoud Ghozlan denied such meetings were taking place.

Wael Ghonim, a former Google executive and secular activist in last year’s revolution, stood with Morsi on Friday. “We are not bartering between security and democracy, between security and freedom,” he said.

Ghonim, who became an icon of the popular revolt that toppled President Hosni Mubarak in February 2011, said he has had serious disagreements with the Brotherhood and that his presence should not be construed as an endorsement.

“We’re in a moment where we have to differentiate between these disagreements and turning against democracy,” Ghonim said. He said he was “not here with the Brotherhood, but for the legitimacy of the vote.”

In Tahrir Square, scene of crucial events in the revolution, Morsi supporters held a defiant rally in support of the candidate. The demonstration, which included many who had been bused in from nearby provinces, remained large and energetic during the blistering daytime hours. As the sun set, the overwhelmingly Islamist crowd set off fireworks and broke into chants denouncing Shafiq.

“Ahmed Shafiq is a thief,” a group chanted.

Others directed their message toward the military chiefs. One man held a sign that said: “Military generals: remember. We removed Mubarak. Stop now. Otherwise, it’s your turn.”