The organizers of anti-government protests that brought millions of Egyptians into the streets this weekend gave Islamist President Mohammed Morsi until Tuesday afternoon to step down or else it will hike up its campaign, as protesters overran and ransacked the headquarters of Morsi’s Muslim Brotherhood.

In a sign of Morsi’s growing isolation, five Cabinet ministers met Monday to consider resigning their posts and joining the protest movement, the state news agency said. The meeting gathered the communications, legal affairs, environment, tourism and water utilities ministers, MENA reported.

The ultimatum issued Monday by Tamarod, the protest organizers, increases pressure on Morsi a day after the opposition’s massive show of force on the streets, with millions packing Cairo’s Tahrir Square, the streets outside the presidential palace and main squares in cities around the country on the anniversary of Morsi’s inauguration.

The main rallies in Cairo were largely peaceful, but deadly violence broke out in several parts of the country, often when marchers came under gunfire, apparently from Islamists. At least 16 people were killed and more than 780 injured, Health Ministry spokesman Yehya Moussa told state television.

Tamarod, Arabic for “Rebel,” issued a statement giving Morsi until 5 p.m. (1500 GMT) on Tuesday to step down and pave the way for early presidential elections or else it would bring the crowds back out, march on more palaces and launch “complete civil disobedience.” Protesters were already gearing up for new rallies Monday.

The group also called on the powerful military and the police to clearly state their support for the protesters. Police mostly stayed on the sidelines Sunday, and some officers have vowed they will not protect the Brotherhood. The army has sent reinforcements to bases on the outskirts of Cairo and other cities across the nation. Its chief, Defense Minister Abdel-Fattah el-Sisisi, had given Morsi and the opposition a week to work out their differences — a deadline that passed Sunday.

The call reflected opposition hopes that a sign from the military could tip the balance against Morsi and avert what could be a destabilizing standoff.

Morsi has said he will not quit, saying that street action must not be allowed to remove an elected president or else the same could happen to future presidents. At the same time, he has offered no concessions — though his opponents appear in no mood to accept anything short of his removal anyway. His Islamist supporters, some of them hard-liners who belong to formerly armed militant groups, have vowed to defend him.

The concern is that violence could escalate with the two sides dug in and anger mounting on both sides. Morsi’s Islamist supporters showed Sunday they were willing to unleash deadly force when protesters approached their positions, with clashes erupting in multiple cities.

In Cairo, protesters Sunday night attacked the Brotherhood’s main headquarters, pelting it with stones and firebombs. Brotherhood backers barricaded inside opened fire on them in clashes that went on for hours and left eight dead. In the early hours Monday, protesters breached the walls of the six-story luxury villa and stormed inside.

They carted off furniture, files, rugs, blankets, air conditioning units and portraits of Morsi, according to an Associated Press journalist at the scene. One protester emerged with a pistol and handed it over to a policeman outside.

Footage on local TV networks showed smashed windows, blackened walls and smoke billowing out of the fortified villa in the Muqatam district in eastern Cairo. A fire was still raging on one floor hours after the building was stormed. One protester tore down the Muslim Brotherhood sign from the building’s front wall, while another hoisted Egypt’s red, black and white flag out an upper-story window and waved it in the air in triumph.

It was not immediately clear whether the Brotherhood supporters holed up inside fled. One witness account spoke of the gunmen running out of the building under the cover of heavy gunfire. Another said they fled through a back door.

Morsi’s critics view the Brotherhood headquarters as the seat of real power in Egypt, consistently claiming that the Islamist group’s spiritual leader, Mohammed Badie and his powerful deputy, Khairat el-Shater, actually call the shots behind Morsi. Morsi and Brotherhood officials have denied this and say they have tried to give opponents a greater voice, only to be spurned.

If the five Cabinet ministers resign, it would show the further paring down of Morsi’s backing outside the Brotherhood. The five are among the non-Islamists in a Cabinet where Brotherhood members hold 11 of more than 30 spots and Brotherhood sympathizers hold several more.

On Monday, anti-Morsi protesters were gearing up for a second day of demonstrations.

Some protesters spent the night in dozens of tents pitched in the capital’s central Tahrir Square and in front of the president’s Ittihadiya Palace. They have vowed to stay there until Morsi resigns. The president’s supporters, meanwhile, continued their sit-in in front of a major mosque in another part of Cairo.

The anti-Morsi demonstrators are calling for widespread labor strikes in an attempt to ratchet up the pressure on the president, but it was not immediately clear whether 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 instead of Ittihadiya.

Sunday’s protests were the largest seen in Egypt in the 2½ years of turmoil since the ouster of autocrat Hosni Mubarak in February 2011.

For weeks, Morsi’s supporters have depicted the planned protest as a plot by Mubarak loyalists to return to power. 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.

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.”

(AP)