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 Mohamed Morsi in the early morning hours.

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 a military building 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 ultra-conservative 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.”

Another Islamist, former presidential candidate Abdel Moneim Aboul Fotouh, who had met with Mansour two days ago, called for him to resign after the violence.

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 Tuesday, creating more potential flash points for conflict.

Bloodshed and panic

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, and sticky pools of blood were visible on the ground at the scene hours after the attack.

Abdel Naguib Mahmoud, a lawyer from the Nile Delta town of Zagazig, said he and fellow protesters had knelt to the pavement for the second time, their backs to the Republican Guard headquarters, when he heard shouts from the perimeter that security forces were encroaching.

“So we finished our prayer rapidly,” Mahmoud said. He said he heard the boom of tear-gas canisters being fired and the crackle of gunfire. Running toward the entrance of the sit-in area, he and several friends began to pick up the wounded, Mahmoud said. More shots rang out, and the men lay down on the pavement.

Mahmoud said he saw forces in military fatigues and police officers dressed in black. Moments later, he said, an officer stood over him and kicked him, telling him to move. When he ran, gunmen opened fire. He said he was hit in the back with birdshot, and he lifted his shirt to reveal a scattering of small bloody wounds.

To support their assertions that they were defending against an assault, military officials played video footage at a news conference that sought to show an increasing ferocity of attack by Brotherhood supporters at the scene.

The footage showed at least one man with a short rifle and another with a handgun firing at soldiers in the daylight after the initial conflict started. The military said armed men on motorcycles had started the assault, but it offered no evidence to back up the assertion.

Individuals in the crowd are shown hurling rocks at the troops and later launching pieces of broken toilet bowls from rooftops and chucking what appear to be spears.

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.

Brotherhood ban sought

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 Tuesday in front of the embassy. Many Morsi opponents view the United States as having sided with the Islamist.

Amro Hassan and Sharaf al-Hourani in Cairo contributed to this report.