Updated 10:36 p.m. ET
CAIRO 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.
Egyptians are "fed up with a president they blame for the country's economic woes and deteriorating security situation, including a spike in murders and sexual assaults," CBS News correspondent Clarissa Ward reported.
Fears were widespread that the two sides could be heading to a violent collision 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. 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.
"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, even as youths clashed with Brotherhood supporters at the site. Three 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.