Updated at 10:04 a.m. Eastern
CAIRO Protesters stormed and ransacked the Cairo headquarters of President Mohammed Morsi's Muslim Brotherhood Islamist group on Monday.
An Associated Press video journalist at the scene said protesters stormed the six-story building in an eastern Cairo district, leaving the heavily fortified villa with furniture and files.
Footage on local TV networks showed smashed windows and smoke billowing out of the building. One protester was seen removing the Muslim Brotherhood sign from the building's front wall.
The storming of the Brotherhood's headquarters followed overnight clashes between armed Morsi supporters barricaded inside the building and young protesters pelting it with firebombs and rocks.
At least 16 people were killed and more than 780 injured in the violence Sunday and early Monday, Health Ministry spokesman Yehya Moussa told state television.
On Sunday, supporters of the Islamist leader barricaded inside the Brotherhood office opened fire on protesters pelting the suburban villa with rocks and firebombs. A fire broke out in the heavily fortified building.
Some protesters spent the night in dozens of tents pitched in the capital's central Tahrir Square and the Ittihadiya presidential palace. They have vowed to stay there until Morsi resigns.
CBS News correspondent Clarissa Ward reported that Tahrir was largely quiet Monday morning, but the demonstrations usually gather steam in the afternoon and early evening. Egyptians were also bracing for another wave of unrest, as the protest movement said it was giving Morsi until Tuesday afternoon to step down, or there would be a fresh round of national civil disobedience.
The anti-Morsi demonstrators are calling for widespread labor strikes to start Monday in an attempt to ratchet up the pressure on the president, but it was not immediately clear how unions would respond to the call. Organizers are also calling for sit-ins at the Cabinet building, interim parliament, and another presidential place where Morsi has been working since late last week.
Sunday's protests were the largest seen in Egypt in the 2 and a half years of turmoil since the ouster of autocrat Hosni Mubarak in February 2011.
Ward said many Egyptians blame Morsi for their country's economic woes and deteriorating security situation, including a spike in murders and sexual assaults since he came to power, but Morsi still enjoys huge support. His backers are also on the streets in Cairo and other cities, and some say they will fight to defend the president.
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 vote deemed free and fair.
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, sticks and shields.
The anti-Morsi protesters aimed to show by sheer numbers that the country has irrevocably turned against him, 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 Tahrir 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 comes next. Protesters have vowed to stay on the streets until he steps down. 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."