But in the wake of the bloody street clashes that took place just outside the sit-in early on Saturday, leaving at least 72 Brotherhood supporters dead and hundreds wounded, another, more embattled language can be heard among the masses gathered around a large outdoor stage. Many Brotherhood members are enraged by the reaction of Christian leaders and the secular elite, who — the Islamists say — seemed to ignore or even endorse the killings while giving full-throated support to calls by Egypt’s defense minister, Abdul-Fattah el-Sisi, for a continued crackdown. 

As the Brotherhood prepares for the possibility that the sit-in will be forcibly dispersed by the police, and that the organization will be driven underground, it faces a crisis that could shape its identity for years to come. For all its stated commitment to democracy and nonviolence, the Brotherhood’s only reliable partners now are other Islamist groups whose members may be more willing to use violent or radical tactics — partners that would tar the Brotherhood’s identity as a more pragmatic movement with a broader base. 

“Now there is just one big Islamist camp on one side and the military on the other, and the differences between the Brotherhood and other Islamists are blurred,” said Khalil al-Anani, an expert on Islamist movements and Egyptian politics at Durham University in England. “It’s a populist confrontation on both sides, driven by hatred.” 

Even the Brotherhood’s own members may prove harder to control after the blood spilled on the weekend. On Saturday, some of the group’s leaders pleaded with young members who were confronting the police and plainclothes assailants to retreat to the relative safety of the sit-in. The leaders were rebuffed, a startling act of insubordination for a group that prides itself on strict hierarchy and iron discipline. 

With much of its leadership — including Mr. Morsi — held incommunicado, the Brotherhood has been unable to conduct any high-level internal dialogue about what to do. Its options are limited in any case, because to back down now, with no guarantee from Egypt’s interim government that the Brotherhood would be spared deeper repression in the future, could be political suicide. Backing down would also violate the group’s understanding of Islamic law, under which no decision to undercut Mr. Morsi can be made without consulting him, according to Gehad el-Haddad, a Brotherhood spokesman. 

In a sense, the Brotherhood’s struggle in recent weeks has been a return to painfully familiar ground. Banned for decades under President Hosni Mubarak and his predecessors, the group grew and matured under the pressure of constant police harassment. Its top leaders were shaped by long years in prison, and many of them were arrested again in early July when the military deposed Mr. Morsi.

Most of the group’s remaining leaders are now effectively confined to the main protest sit-in, in a broad intersection around a towering white mosque in a residential area of northeast Cairo known as Nasr City. On any given evening, some of them can be found in one of the mosque’s outbuildings, looking exhausted but focused as they move from one crisis meeting to the next. 

In some ways, Brotherhood members say, the current crisis is almost comforting. Gone are the challenges and inevitable compromises of governing the country, which eroded the group’s popularity over the past year. Now it is in opposition again, a role that sits more easily with its historical self-image as a bulwark against oppression. 

“These people dare to mock our religion!” shouted Safwat Hegazy, a Brotherhood leader, as he stood under the bright stage lights on Saturday night and the flag-waving crowd roared its approval. “God will punish them,” he continued. A chant went up in the crowd: “The people want the trial of the serial killer!” — a reference to General Sisi. 

The sit-in, like many of the Arab protests of 2011, has taken on elements of a carnival: fruit and popcorn vendors push carts through the crowds, and visitors on their way to the stage clamber over sleeping bodies. The morning and evening meals of the fasting month of Ramadan, handed out in plastic-wrapped foil packages by Brotherhood volunteers, impose a ritual congeniality. 

But the slurry of garbage underfoot grows thicker every day, and the smell gets worse. Last week the Brotherhood paid for flowers and apologies to be sent to thousands of local residents. 

A core group of Brotherhood leaders who have not been arrested — about a dozen men — meet daily at the sit-in to discuss tactics, Mr. Haddad said during a late-night interview at the meeting room behind the mosque. “They go around, each one presenting his analysis of the situation; then they narrow it down to three or four options, and they vote,” Mr. Haddad said. “Sometimes it’s very heated, with shouting; sometimes it’s easy.” 

The discussions center on tactics like the route and timing of protest marches, he said. Broader discussions of strategy are impossible, given the absence of so many top leaders. 

The mood is “very angry,” Mr. Haddad said. “The military needs to be taught a lesson. At this point it’s a zero-sum game: it’s either the Brotherhood or the old regime. Everyone else is too small to matter.” 

Yet the other Islamist groups, which not long ago vied with the Brotherhood for electoral seats, are now important parts of its effort to restore Mr. Morsi to power. Although one powerful Islamist group, the ultraconservative party Al Nour, officially supported the military’s move, many of its rank and file sided with the Brotherhood and can now be found at the sit-in. 

Many Islamists from a variety of factions seem to believe that if the Brotherhood falls, they — and the cause of political Islam here and abroad — will fall with it. 

In a tent at the Nasr City sit-in, members of Gamaa al-Islamiya, which carried out a campaign of terrorism in Egypt before renouncing violence more than a decade ago, sat together on the thin mats covering the pavement, where they sleep every day during the long hours of fasting for Ramadan, the Muslim holy month. 

“What is strange is that we followed the democratic game very well,” said Yahya Abdelsamia, a middle-aged man with the bushy, unkempt beard favored by the ultraconservative Islamists known as Salafis. “We joined the elections, we did what they wanted us to. Then we’re faced with military force.” He added in English, with a pained smile, “Game over.” 

A younger man named Tareq Ahmad Hussein spoke up: “Many of the youth now say, ‘No more ballot boxes.’ We used to believe in the caliphate. The international community said we should go with ballot boxes, so we followed that path. But then they flip the ballot boxes over on us. So forget it. If ballot boxes don’t bring righteousness, we will all go back to demanding a caliphate.” He referred to a system where supreme Islamic religious leaders also held sway over secular life.

A third man said the crisis had been useful in some ways. “It has been a tough test, but it has had benefits — now we know who our true friends are,” he said. “The liberals, the Christian leaders, they stood with the old regime. It was painful to see some fellow Muslims going against us at first, but they have now seen their mistake and returned to us. The Islamic path is clear.” 

The Brotherhood has made some effort to restrain that kind of talk. On a recent evening, an older man in traditional dress was angrily shouting to a reporter about a “war against Islam” led by liberals and the military, and the need for all Muslims to fight against it. Several Brotherhood members urged the man to change his tone, telling him to stick to the words “democracy” and “legitimacy,” and then tried to escort the reporter away.

But the countercurrent cannot be airbrushed away. At the field hospital where dead and wounded Brotherhood supporters were brought during Saturday morning’s fighting, one young Islamist shouted that Christian snipers had been targeting his “brothers” from the rooftops.