CAIRO -- Hundreds of thousands of protesters demanding the ouster of Egypt's president, Mohamed Morsi, poured into the streets of the capital and cities across the country Sunday, while tens of thousands of his Islamist allies gathered with makeshift clubs, helmets and shields vowing to defend the presidential palace.
In an outpouring of rage late Sunday night, a core of demonstrators set fire to the headquarters of Mr. Morsi's Muslim Brotherhood movement in Cairo. Hundreds of attackers with Molotov cocktails vowed to kill anyone inside, and used green pen lasers to search for figures in the windows. "Their leaders left them like sheep for the slaughter," one attacker said. Some Brotherhood members inside fired birdshot at the attackers, wounding several, but there did not appear to be a concerted effort to repel them.
As dusk fell, the massive crowds that gathered in Tahrir Square and outside the palace were larger than any seen here since the removal of Mr. Morsi's predecessor, Hosni Mubarak, on Feb. 11, 2011. Security officials said at least five people -- one in Beni Suef, a Nile Delta town, and four in Assiut in southern Egypt -- were killed, and hundreds more injured in clashes across the country.
Though angry about the lack of public security and desperate economic conditions, the demonstrators were united in the conviction that Mr. Morsi had failed to transcend his personal roots in the Muslim Brotherhood, an insular Islamist movement that is considered Egypt's most formidable political force. The sheer scale of the demonstrations across the country delivered a sharp rebuke to the group's claim to speak for the majority of Egyptians.
"He was of the revolution," said Magdi Morsi, an airline flight planner demonstrating in front of the palace who is not related to the president. He said he had voted for Brotherhood candidates for Parliament as well as for Mr. Morsi but had turned against them for failing to deliver on their promises. "I decided he was a big liar," he said. "He must leave. The public is against him now."
Among demonstrators in the pro-Morsi camp, there were anxieties about their small numbers relative to the size of other crowds shown on television screens. "Have you seen the numbers in Qalyubia, in Dakahleya, in Ittihadeya, in Tahrir?" one Morsi supporter shouted at others sitting on the periphery of their rally, naming some of places where opponents were outnumbering them. "And you are just sitting on the sidewalk! Get up!"
The magnitude of the demonstrations emboldened the protesters, who expressed a growing conviction as the night wore on that nothing less than the fall of the government would suffice. Few talked about the possibility of reforms -- perhaps the only concession they were likely to win -- foreshadowing confrontations to come.
"Morsi has thick skin," said Amr Abdulhakim, who protested near the palace with his family. "He's not leaving today, tomorrow, or in five days. There will be martyrs."
There were scattered calls throughout the protests for intervention by the military, a controversial prospect that has divided Mr. Morsi's opponents. "Come on Sisi," went one chant, referring to the defense minister, Gen. Abdul-Fattah el-Sisi. "Make a decision!"
Mr. Morsi's overt reliance on the Brotherhood's civilian members to defend his authority -- there was a near total absence of police or military security on the streets -- only appeared to underscore his isolation within the Egyptian government as well as among the population. But the ultimate consequences of the outpouring were uncertain. Although many vowed to stay in the streets until Mr. Morsi resigned, there is no legal mechanism to remove him until the election of a new Parliament, and even some critics of Mr. Morsi acknowledge that forcing the first democratically elected president from power would set a precedent for future instability.
The demonstrations remained nonviolent at least until nightfall, defying the pattern of recent days as well as the predictions -- and preparations -- of both sides. But by 9 p.m., a group of protesters calling for Mr. Morsi's removal had attacked the Muslim Brotherhood's headquarters, breaking windows and setting fire to a guard booth.
Jitters about a broader confrontation in Cairo were increasing, in part because of the near total absence of security. And Mr. Morsi's Islamist supporters, in particular, appeared to bracing for a fight.
Many had brought batons, pipes, bats and other makeshift clubs, hard hats or motorcycle helmets and even woks and scraps of metal to use as shields. Contingents of Brotherhood members arriving together formed lines, stood at attention with clubs raised, marched together, or chanted their willingness to bleed for their cause. "We will sacrifice our lives for religion," some chanted. "Morsi's men are everywhere."
A voice over a loudspeaker called out: "We will liberate Egypt tonight. We will cleanse Egypt tonight."
Ayman Saeed, 30, one of the Islamist demonstrators, noted that the police had already failed to protect several Brotherhood offices from destruction in recent days. "We had to do this because we know that the police would not protect us," he said. "You see what is happening in the provinces? That isn't freedom of expression. That's thuggery."
The normally clogged streets of Cairo were all but deserted Sunday, the beginning of the workweek, as businesses closed and residents took refuge. The airports were packed with people trying to get out of the city and worried about a potential shutdown.
Outside Cairo, protesters blockaded or chained shut government offices in the capitals of several provinces across the Nile Delta, state media reported. Protesters also blocked several transit ways, including the major highways and railways connecting Cairo and Alexandria.
Officials of the Muslim Brotherhood said several offices had been set fire or ransacked overnight, adding to a series of such attacks in recent days. More than half a dozen people have died and several hundred have been injured in recent days during skirmishes leading up to Sunday's protests.
The police are in more or less open revolt against President Morsi. In recent days, Egyptian news organizations have posted video footage of armed and uniformed police officers joining street fights, seemingly against members of the Brotherhood defending an Alexandria office.
When protesters closed the highway connecting the Delta towns of Monsoura and Damietta on Sunday, state media reported that a group of police officers joined the blockade, chanting, "Revolution until victory!" and "The people and the police are one hand!"
The military, which seized power for more than a year after Mr. Mubarak's ouster, has tried to strike a tone of neutrality, urging reconciliation of the feuding political factions, but at the same time declaring itself ready to intervene to protect the national security.
In an interview with The Guardian published Sunday, Mr. Morsi acknowledged for the first time that he was not told ahead of time about a statement issued last week by the defense minister, General Sisi, in which he urged the president and his opponents to come together before Sunday's protests and said the military would "intervene to keep Egypt from sliding into a dark tunnel of conflict, internal fighting, criminality, accusations of treason, sectarian discord and the collapse of state institutions."
Many in the opposition saw the statement as a sign of distance between the generals and the president, or as an indication that if Sunday's protests were large enough -- or perhaps violent enough -- the military would take over once again. Cheers rose from the crowd when military helicopters circled Tahrir Square on Sunday.
Mr. Morsi said he retained full confidence in General Sisi. "We constantly talk together over time," Mr. Morsi said in the Guardian interview, but "we can't restrict every single word announced by officials in this country."
Mr. Morsi said the military had suffered during its time leading the country after Mr. Mubarak and had no desire to return. "They're busy now with the affairs of the army itself," Mr. Morsi said.
As protests began Sunday, however, the military continued to give out ambiguous signals. "All units" were at "high alert," ready "to move 'on the moment' when something urgent or a threat to national security occurs," Egyptian state media reported, citing an unidentified military source.
"The official military source clarified that the most recent instructions given to soldiers and officers is to protect the will of the people without bias to any side at the expense of the other, especially as the political forces have not reached any formula of consensus," the Web site of the flagship state newspaper, Al Ahram, reported.
In recent weeks, Mr. Morsi's government has extended and refortified the walls of the presidential palace, which serves as his office, in anticipation of an attack during Sunday's protests. During the interview with The Guardian, Mr. Morsi was working out of an alternate facility, Quba Palace, the birthplace of former King Farouk. State media reported that the prime minister, Hisham Qandil, was also working out of an alternate office for security reasons.
In the same interview, Mr. Morsi acknowledged more explicitly than before that he had erred in his "constitutional declaration" last fall putting his own power above the authority of the courts until the passage of a constitution. Although he said he meant to block the courts from dissolving the assembly drawing up the constitution, the move struck many as authoritarian, and it set off the waves of protests that built up to Sunday's demonstrations.
"It contributed to some kind of misconception in society," Mr. Morsi said in the interview. His advisers have privately acknowledged deep regrets over the declaration, although they argue that subsequent rulings have confirmed that the high court -- full of justices appointed from Mr. Mubarak -- was indeed poised to dissolve the assembly.
But Mr. Morsi repeatedly blamed the old Mubarak bureaucracy and elites -- "the deep state and the remnants of the old regime" -- for the protests. He insisted that those who profited under Mr. Mubarak had paid hired thugs to attack his supporters and the Muslim Brotherhood.
"They have money, and they got this money from corruption," he said. "They used this corrupt money to pull back the regime, and pull back the old regime into power. They pay this corrupt money to thugs, and then violence takes place."
And he argued that if he gave in to the demands for his exit, it would only prolong the chaos of Egypt's transition. If an elected president resigned under pressure from the street, "there will be people or opponents opposing the new president too, and a week or a month later, they will ask him to step down," Mr. Morsi said.
He insisted that he was still "very" confident that he would serve out his term as Egypt's first democratically elected head of state.
"It has been a difficult, very difficult year, and I think the coming years will also be difficult," Mr. Morsi said in the interview. "But I hope that I will all the time be doing my best to fulfill the needs of the Egyptian people."
Still, Alaa al-Aswany, a prominent Egyptian writer and critic of the Brotherhood who turned out to demonstrate, said the public had rendered its verdict Sunday. "I know the Egyptian people," he said. "They never let us down. Enough is enough.
"It has been decided for Mr. Morsi," he continued. "We are waiting for him to understand."
Mayy el Sheikh contributed reporting.
This article originally appeared in The New York Times.