In a sign of Morsi's growing isolation, five Cabinet ministers said they have resigned, the state MENA news agency reported. The five were the ministers of communications, legal affairs, environment, tourism and water utilities. The foreign minister also submitted his resignation, government officials said, speaking on condition of anonymity because they were not authorized to talk to the press. The governor of the strategic province of Ismailia on the Suez Canal, Hassan el-Rifaai, also quit.
Sunday's protests on the first anniversary of Morsi's inauguration were the largest seen in the country in the 2½ years of turmoil since Egyptians first rose up against Mubarak in January 2011. Millions packed Tahrir Square, the streets outside the Ittihadiya presidential palace and main squares in cities around the country.
Violence broke out in several parts of the country, often when marchers came under gunfire, apparently from Islamists. In Cairo, anti-Morsi youth attacked the main headquarters of the Muslim Brotherhood with stones and fire bombs, while Brotherhood supporters barricaded inside opened fired on them. The clash ended early Monday when the protesters broke into the luxury villa and ransacked it, setting fires.
Nationwide, at least 16 people were killed Sunday and more than 780 injured, Health Ministry spokesman Yehya Moussa told state television.
Earlier, the group organizing the anti-Morsi protests, Tamarod, Arabic for "Rebel," issued an ultimatum of its own, giving Morsi until Tuesday afternoon to step down or it would escalate the rallies.
Under a framework drawn up by Tamarod, after Morsi steps down, the head of the Supreme Constitutional Court would become an interim president and a technocrat government would be formed. An expert panel would write a new constitution to replace the one largely drafted by Islamists, and a new presidential election would be held in six months.
For Islamists, however, the idea of Morsi stepping down is an inconceivable infringement on the repeated elections they won since Mubarak's fall, giving them not only a longtime Brotherhood leader as president but majorities in parliament.
Morsi and Brotherhood officials say they are defending democratic legitimacy and some have depicted the protests as led by Mubarak loyalists trying to return to power. But many of his Islamist allies have also depicted it as a fight against Islam.
"The military has sacrificed legitimacy. There will be a civil war," said Manal Shouib, a 47-year-old physiotherapist at the pro-Morsi rally outside the Rabia al-Adawiya Mosque not far from Ittihadiya.
Outside the palace, protesters contended that Morsi could not survive with only the Islamist bloc on his side.
"It is now the whole people versus one group. What can he do?" said Mina Adel, a Christian accountant. "The army is the savior and the guarantor for the revolution to succeed."
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