Hundreds of thousands thronged the streets of Cairo and cities around the country Sunday and marched on the presidential palace, filling a broad avenue for blocks, in an attempt to force out the Islamist president with the most massive protests Egypt has seen in 2½ years of turmoil.

In a sign of the explosive volatility of the country's divisions, young protesters mainly from the surrounding neighborhood pelted the main headquarters of President Mohammed Morsi's Muslim Brotherhood with stones and firebombs, and at one point a fire erupted at the gates of the walled villa. During clashes, Brotherhood supporters barricaded inside opened fire on the attackers, and activists said at least five protesters were killed.

At least five more anti-Morsi protesters were killed Sunday in clashes and shootings in southern Egypt.

Fears were widespread that the collisions between the two sides could grow more violent in coming days. Morsi made clear through a spokesman that he would not step down and his Islamist supporters vowed not to allow protesters to remove one of their own, brought to office in a legitimate vote. During the day Sunday, thousands of Islamists massed not far from the presidential palace in support of Morsi, some of them prepared for a fight with makeshift armor and sticks.

The protesters aimed to show by sheer numbers that the country has irrevocably turned against Morsi, a year to the day after he was inaugurated as Egypt's first freely elected president. But throughout the day and even up to midnight at the main rallying sites, fears of rampant violence did not materialize.

Instead the mood was largely festive as protesters at giant anti-Morsi rallies in Cairo's central Tahrir Square and outside the Ittihadiya palace spilled into side streets and across boulevards, waving flags, blowing whistles and chanting.

Fireworks went off overhead. Men and women, some with small children on their shoulders, beat drums, danced and sang, "By hook or by crook, we will bring Morsi down." Residents in nearby homes showered water on marchers below — some carrying tents in preparation to camp outside the palace — to cool them in the summer heat, and blew whistles and waved flags in support.

"Mubarak took only 18 days although he had behind him the security, intelligence and a large sector of Egyptians," said Amr Tawfeeq, an oil company employee marching toward Ittihadiya with a Christian friend. Morsi "won't take long. We want him out and we are ready to pay the price."

The massive outpouring against Morsi raises the question of what is next. Protesters have vowed to stay on the streets until he steps down, and organizers called for widespread labor strikes starting Monday. The president, in turn, appears to be hoping protests wane.

For weeks, Morsi's supporters have depicted the planned protest as a plot by Mubarak loyalists. But their claims were undermined by the extent of Sunday's rallies. In Cairo and a string of cities in the Nile Delta and on the Mediterranean coast, the protests topped even the biggest protests of the 2011's 18-day uprising, including the day Mubarak quit, Feb. 11, when giant crowds marched on Ittihadiya.

It is unclear now whether the opposition, which for months has demanded Morsi form a national unity government, would now accept any concessions short of his removal. The anticipated deadlock raises the question of whether the army, already deployed on the outskirts of cities, will intervene. Protesters believe the military would throw its weight behind them, tipping the balance against Morsi.

The country's police, meanwhile, were hardly to be seen Sunday. In the lead-up to Sunday, some officers angrily told their commanders they would not protect the Brotherhood from protesters, complaining that police are always caught in the middle, according to video of the meeting released online.

"If the Brothers think that we will give up and leave, they are mistaken," said lawyer Hossam Muhareb as he sat with a friend on a sidewalk near the presidential palace. "They will give up and leave after seeing our numbers."

Violence could send the situation spinning into explosive directions.

The fire at the Brotherhood headquarters, located on a plateau overlooking Cairo, sent smoke pouring in the air. Witnesses said it was caused when the youths hurled a gas canister at the heavily barricaded gate and it exploded. For several hours after, Brotherhood supporters inside fired on stone-throwing youths outside. At least five on the anti-Morsi side were shot to death, and 60 were wounded, an activist who monitored casualties at the hospital, Nazli Hussein, said.

Southern Egypt saw deadly attacks on anti-Morsi protests, and five people were killed. Two protesters were shot to death during clashes outside offices of the Brotherhood's Freedom and Justice Party, one in Beni Suef, the other in Fayoum.

In the city of Assiut, a stronghold of Islamists, gunmen on a motorcycle opened fire on a protest in which tens of thousands were participating,, killing one person, wounding four others and sending the crowd running.

The enraged protesters then marched on the nearby Freedom and Justice offices, where gunmen inside opened fire, killing two more, security officials said, speaking on condition of anonymity because they weren't authorized to talk to the press. Clashes erupted, with protesters and security forces fighting side by side against Morsi's supporters.

At least 400 people were injured nationwide, the Health Ministry said.

Morsi, who has three years left in his term, said street protests cannot be used to overturn the results of a free election.

"There is no room for any talk against this constitutional legitimacy," he told Britain's The Guardian newspaper in an interview published Sunday, rejecting early elections.

If an elected president is forced out, "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," he said.

Morsi was not at Ittihadiya as Sunday's rally took place — he had moved to another nearby palace.

As the crowds massed, Morsi's spokesman Ihab Fahmi repeated the president's longstanding offer of dialogue with the opposition to resolve the nation's political crisis, calling it "the only framework through which we can reach understandings."

The opposition has repeatedly turned down his offers for dialogue, arguing that they were for show.

The demonstrations are the culmination of polarization and instability that have been building since Morsi's June 30, 2012, inauguration. The past year has seen multiple political crises, bouts of bloody clashes and a steadily worsening economy, with power outages, fuel shortages, rising prices and persistent lawlessness and crime.

In one camp are the president and his Islamist allies, including the Muslim Brotherhood and more hard-line groups. Morsi supporters accuse Mubarak loyalists of being behind the protests, aiming to overturn last year's election results, just as they argue that remnants of the old regime have sabotaged Morsi's attempts to deal with the nation's woes and bring reforms.

Hard-liners among them have also given the confrontation a sharply religious tone, denouncing Morsi's opponents as "enemies of God" and infidels.