Wounded supporters of Mr. Morsi were treated in a makeshift hospital. At least 51 civilian demonstrators were killed and more than 400 were wounded, almost all of them by gunfire, health officials said. More Photos »

CAIRO — The mass shooting of Islamist protesters by security forces on Monday at a sit-in for Mohamed Morsi, the ousted president, injected new outrage into the standoff over his removal by Egypt’s top generals, darkening hopes that they might reconcile the polarizing forces that have torn at the fabric of the country.

It was by far the deadliest day of violence since the revolt that overthrew former President Hosni Mubarak in early 2011. Within a few hours around dawn, advancing soldiers and police officers killed at least 51 civilians and wounded more than 400, almost all hit by gunfire, health officials said.

Army and police spokesmen said that one soldier and two policemen had also been killed. But according to witnesses and video footage, one of the policemen appeared to have been shot by soldiers, and the military provided little evidence to back its claim that the fighting had been instigated by the Islamists.

The scale and nature of the killings drove a deeper wedge between Mr. Morsi’s Islamist backers and their opponents, and diminished the chances that his allies in the Muslim Brotherhood might soon be coaxed back into a political process that they deem illegitimate after the military overthrew the elected president.

At the same time, the bloodshed sharpened a fierce debate about whether the new military-led interim government that replaced Mr. Morsi last week was moving toward a democracy or away from it. Two and a half years after the overthrow of Mr. Mubarak, the institutions, tactics and dynamics of the decades-old secular-authoritarian government seemed, at least for the moment, to snap back into place.

Some who vehemently denounced Mr. Mubarak’s use of brute force to silence critics were far more tepid about criticizing the killings of Mr. Morsi’s supporters, calling only for an inquiry to determine the root cause. The United States, which has conspicuously not condemned Mr. Morsi’s ouster, was also mild, calling on security forces to exercise restraint.

“Violence begets violence and should be strongly condemned,” Mohamed ElBaradei, the former diplomat and liberal leader, said in a statement on Twitter. “Independent investigation is a must.”

By contrast, Essam el-Erian, a senior Brotherhood leader, called the killings “an outright massacre” by “a fascist coup government.”

Leaders of the Brotherhood, Egypt’s largest Islamist group and best-organized political force, said the generals had now shown their authoritarian colors, using lethal weapons to crush dissent while holding the freely elected president captive. They called for a national “uprising” against the return of a military dictatorship.

Al Nour, the only Islamist party that had backed the military’s takeover, suspended its participation in the interim government, accelerating the disintegration of Egyptian politics toward a culture war between Islamists and their foes.

The armed forces, on the other hand, claimed that Mr. Morsi’s supporters had attacked them first with rocks, gunfire and army-issued tear gas bombs, though dozens of witnesses — including some of Mr. Morsi’s opponents — disputed that account.

At a news conference, Ahmed Ali, a military spokesman, said the security forces had responded with rubber bullets and gas bombs after coming under attack by heavy gunfire. He addressed a pointed question about human rights to Western critics: “What human rights are there for an armed person who terrorizes citizens and attacks military establishments?”

The police, who had never fully accepted Mr. Morsi’s authority, reveled in the day and sought to revise history: a spokesman contended that the Muslim Brotherhood — and not the police — had been responsible for killing protesters during the revolt against Mr. Mubarak. “Policemen never thought that history would speak so quickly to prove the complete innocence of the policemen in the events of the January 2011 revolution,” said the spokesman, Hany Abdel Lateef.

Some also suggested that Mr. Morsi’s supporters might be to blame for the fighting.

“We expect violent actions from the side of the Muslim Brotherhood, and we cannot accept that armed gatherings be labeled as peaceful protests or sit-ins,” Khalid Talima, a representative of the coalition formed around the anti-Morsi protests that preceded his ouster, said at a news conference under the banner “Muslim Brotherhood-American conspiracy against the revolution.”

Seeking to capitalize on the killing to rally supporters, the Muslim Brotherhood claimed that the soldiers had killed women and children. But hospitals and morgues reported no such casualties.

The violence began around 4 a.m., as hundreds of Islamists were observing dawn prayers at a vigil for Mr. Morsi outside the presidential guard facility where he was believed to be detained. What set off the violence could not be determined.

In addition to the official statements from the army and the police, one neighbor cited by The Associated Press said in an account posted online that she had run to a window when she heard gunfire and had seen men shooting at security forces from a mosque roof. But that neighbor, identified as Mirna al-Hikbawy, wrote that she had not seen the start of the fighting.

Others, including both supporters and opponents of Mr. Morsi, said the military and the police had fired with little or no provocation, unloading tear gas, birdshot and bullets.

“They opened fire on us while we were praying,” Moataz Abu al-Shakra, a 25-year-old electrical engineer, said, huddled behind a sheet of corrugated metal that Mr. Morsi’s supporters had sought to use as a shield. The metal was riddled with bullet holes, and he pointed to two pools of blood on the ground.

“It is like they were fighting a war between two countries, not like our army or police,” he said. “They are criminals.”

Sit-in participants said gunmen had fired on them from atop the military buildings surrounding their camp. Video footage captured by the Islamists showed a soldier firing down from a roof while another calmly filmed the mayhem below.

Sandbagged gun turrets were still visible hours later on some rooftops, and the angles of scores of bullet holes in cars, lampposts and the Islamists’ makeshift metal barriers indicated that gunfire hit at an angle from above.

Many witnesses said the fighting lasted for hours, with hundreds of heavily armed soldiers chasing mostly unarmed protesters through the streets for blocks while continuing to shoot. Bullet holes, bullet casings and pools of blood dotted the ground hundreds of yards from the presidential guardhouse where the fighting had begun.

The pro-Morsi demonstrators attempted to fight back by throwing rocks, and they tried to build barricades against the bullets. Two witnesses said they had seen at least two of Mr. Morsi’s supporters armed with what the witnesses described as primitive shotguns. Egyptian state television showed footage of what it described as a pro-Morsi fighter firing a primitive shotgun at advancing soldiers about 250 yards from the initial shooting. In another video clip on state television, a man in a black mask was seen walking with a similar weapon. Many of the videos were set in broad daylight.

Ibrahim el-Sheikh, a neighbor and a relative of a New York Times employee in Cairo, said that one of the police officers, Mohamed el-Mesairy — cited by the police spokesman as a “martyr” — appeared to have been killed by the advancing soldiers.

Mr. Mesairy was a familiar neighborhood policeman who had sought to hide in his parked car as Mr. Morsi’s supporters fled past, according to Mr. Sheikh and video footage he captured from his window. The video showed the gunfire of the advancing police and soldiers hitting the side of the car, and the bullet holes were visible hours later.

Mr. Sheikh, who signed a petition and joined protests for Mr. Morsi’s ouster, said he and others carried the officer’s body from his car. “He did not have a head anymore,” he said.

Gomaa Gaber, a 53-year-old mechanic with a bloodstained shirt, said he threw himself onto a younger relative, Ali Mohamed Said, 24, to protect him. But Mr. Said had already been shot in the chest. “He died in my arms,” Mr. Gaber said.

At the Nasr City hospital, a few minutes’ drive from the initial shooting, Dr. Bassem al-Sayed, a surgeon, said he had seen a similar scene only once before, around Jan. 25, 2011, when Egyptians began their revolt against President Mubarak. “This is worse,” he said.

At the news conference, the military spokesman showed video footage of handguns, tear gas grenades and bottles of whiskey he said the soldiers had found in the Islamists’ tents.

As the conference began, a crowd of Egyptian journalists demanded that a television crew from Al Jazeera leave. Most Egyptian journalists in both the state and private media believe that Al Jazeera, the pan-Arab network owned by the government of Qatar, sympathizes with the Muslim Brotherhood.

“We are in Egypt, the country of democracy,” Mr. Abdel Lateef, the police spokesman, said to raucous cheers as the crew left.

Mr. Ali, the military spokesman, raised alarms about the Arab Spring itself — heresy here just a few months ago.

He called Islamist charges the military had massacred demonstrators a new kind of “information warfare” that “runs through the Middle East region and we see since the breaking of the Arab Spring revolutions.”

“They’re all wars against the state by its own citizens,” he said, “and the main weapon in these wars is the circulation of strife, rumors and lies.”