CAIRO — Egypt lurched into dangerous new terrain Monday as an angry and bloodied Muslim Brotherhood called for an "uprising" against the new order, and the head of Egypt's top Islamic authority warned that the country was headed toward civil war after security forces opened fire on supporters of ousted President Mohammed Morsi.
In one of the deadliest days of political violence since Hosni Mubarak was overthrown more than two tumultuous years ago, Egyptian soldiers on Monday fired on protesters as they massed in front of the military headquarters, where they believe Morsi — ousted by the military on Wednesday — is being held under house arrest, according to witnesses and security officials.
A Health Ministry spokeswoman said 51 people were killed and 435 were wounded in the shootings. Military officials said that they responded after being fired upon by protesters and that one soldier was killed and 42 were injured.
Interim President Adly Mansour issued a decree late Monday that set the parameters for a referendum on a revised constitution within about 41/2 months, parliamentary elections within about six months and presidential elections after that.
The measures appeared aimed at lending some stability to a situation that threatened to spiral out of control. But a prime ministerial appointment that had been expected Monday never came, and the day was consumed with news of the violence and an immediate debate about its causes and meaning. Both the military establishment and the Muslim Brotherhood pleaded their cases to the Egyptian people, each swearing it was the innocent victim.
Islamist witnesses, including many members of the Muslim Brotherhood, said the shootings started unprovoked as protesters were reciting dawn prayers in front of Cairo's Republican Guard headquarters.
Security officials said members of the pro-Morsi camp attacked first.
"We did not attack protesters; we were rather defending a military facility," said Ahmed Ali, a spokesman for the military. "They moved on us to provoke our soldiers and create this violent scene."
Regardless of who fired the first shots, the violence shocked Egyptians and threw the nation's shaky post-coup order into further disarray, as important factions pulled out of the coalition that lent broad unity to the effort to oust Morsi, who led the country for 368 days.
The ultraconservative Salafist Nour party, the only Islamist political bloc to support Morsi's ouster, said it would abandon negotiations over who should take over as prime minister to protest what it called a "massacre."
Sheik Ahmed el-Tayeb of al-Azhar, Egypt's top Islamic authority, had expressed support for Morsi's ouster. But Monday, he appeared on state television and said he would remain in seclusion at his home "until everybody takes responsibility to stop the bloodshed, to prevent the country from being dragged into a civil war."
The Muslim Brotherhood's political arm urged "an uprising," using the language of the Palestinian struggle against Israel.
Dramatic funeral marches were expected by the dozens today, creating more potential flash points for conflict.
The violence on Monday started before dawn, witnesses said, and continued as the sun rose above Cairo. Morsi supporters said they had been praying when the tumult began.
Witnesses described a scene of panic, with live fire, birdshot and tear gas seemingly coming at them from all directions. A doctor directing a field hospital for the wounded said many of the dead had gunshot wounds to the head and back.
The Muslim Brotherhood's Freedom and Justice Party issued a statement calling for an "uprising against those who want to steal the revolution with tanks" and asking the world to prevent a "new Syria."
At an emotional news conference at the Rabaa al-Adawiya Mosque, where Morsi supporters have camped since he was deposed last week, a doctor and others said protesters had been shot in the back as they knelt to pray.
Meanwhile, the main Tamarod activist group, which organized the massive protests last week that led to Morsi's removal, called for the Brotherhood's political wing to be dissolved and its leadership barred from political life.
That treatment, Tamarod said on Twitter, would echo the ban placed on former president Hosni Mubarak's political party after the 2011 Egyptian revolution. A ban on the Brotherhood and other religious parties also would fall in line with Mubarak's policy, under which many of the Brotherhood's leaders spent decades moving in and out of prison.
The U.S. Embassy in Cairo announced Monday that it would be closed to the public Tuesday. Later in the day, popular television host Tawfiq Okasha called for a peaceful protest today in front of the embassy. Many Morsi opponents view the United States as having sided with the Islamist.