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.