CAIRO -- The Egyptian authorities unleashed a ferocious attack on Islamist protesters early Saturday, killing at least 65 people in the second mass killing of demonstrators in three weeks and the deadliest attack by the security services since Egypt's uprising in early 2011.
The attack provided further evidence that Egypt's security establishment was reasserting its dominance after President Mohamed Morsi's ouster three weeks ago, and widening its crackdown on his allies in the Muslim Brotherhood. The tactics -- some victims were killed with single gunshot wounds to the head -- suggested that Egypt's security services felt no need to show any restraint.
"They had orders to shoot to kill," said Gehad el-Haddad, a Brotherhood spokesman. The message, he said, was, "This is the new regime."
The killings came a day after hundreds of thousands of Egyptians marched in support of the military, responding to a call by its commander for a "mandate" to fight terrorism. The appeal by Gen. Abdul-Fattah el-Sisi, who has emerged as Egypt's de facto leader since the military removed Mr. Morsi from power, was widely seen as a green light to the security forces to step up their repression of the Islamists.
In the attack on Saturday, police officers were joined by civilians in firing live ammunition at the protesters. With hundreds of people gravely wounded, the toll seemed certain to rise, and by Saturday evening had already surpassed the more than 60 deaths on July 8, when soldiers and police officers fired on pro-Morsi demonstrators.
As the death toll has mounted, hopes have faded for a political solution to the standoff between the military and the Brotherhood, whose leaders are imprisoned or preparing themselves for jail.
In a televised news conference hours after the clash, the interior minister, Mohamed Ibrahim, absolved his men of any responsibility. His officers, Mr. Ibrahim said, "have never and will never shoot a bullet on any Egyptian."
He blamed the Brotherhood for the deaths, referring to it as "those who preach and incite violence." And he suggested that further repression was imminent as the authorities prepared to break up sit-ins that thousands of Mr. Morsi's supporters have held for weeks.
Mr. Ibrahim said he hoped the protesters would be "reasonable" and remove themselves voluntarily to avoid further bloodshed.
"We all hope and want the sit-ins to be broken up now, but blood is precious for us as well," he said. "Be sure that dispersing the sit-in with force will lead to losses."
Mohamed ElBaradei, the Nobel laureate who serves as vice president in the interim government, added a rare note of support for the Brotherhood from the country's new leaders, writing on Twitter that he condemned the "excessive use of force" and was trying to "end the standoff in a peaceful manner."
In Washington, Secretary of State John Kerry called on Egypt's leaders to "to respect the right of peaceful assembly and freedom of expression" and "to help their country take a step back from the brink." In a statement, he urged an independent inquiry into the violence and an inclusive political dialogue.
"This is a pivotal moment for Egypt," he said. "Over two years ago, a revolution began. Its final verdict is not yet decided, but it will be forever impacted by what happens right now."
Secretary of Defense Chuck Hagel spoke by telephone with General Sisi, urging him to exercise restraint and "take steps to prevent further bloodshed and loss of life," according to a Pentagon statement.
The violence broke out on Friday night after a day of large, competing marches by supporters of Mr. Morsi and his opponents expressing solidarity with the military. At least eight people died on Friday, but there was not the kind of widespread violence that many had feared after General Sisi's speech last Wednesday calling for demonstrations in support of the military.
That changed around 10:30 p.m., when groups of Mr. Morsi's supporters left their vast encampment in Nasr City, marching toward the central October 6 Bridge, where police officers were stationed, according to witnesses. Several people said that the protesters left the camp because it had become overcrowded, and that people had fanned out from the encampment along several boulevards. Others said they planned to march through a nearby neighborhood.
The group that came under attack walked down Nasr Street, past the reviewing stand where former President Anwar el-Sadat was assassinated in 1981, and the pyramid-shaped memorial to the unknown soldier across the street, toward the bridge.
"We didn't have any weapons," said Mohamed Abdulhadi, who said he joined the march, which was "not violent." More than 10 other witnesses confirmed his assertion.
The Interior Ministry released a video after the killings that it said showed Morsi supporters firing birdshot at the police and damaging property. It showed protesters throwing rocks, unidentified people wandering into traffic, and one man pulling out what appears to be a silver pistol and firing it, though it is not clear who the man is, or which side of the fighting he was on.
Mohamed Saeed, a 27-year-old agricultural engineer, said he and some of the other protesters started to exchange words with the officers before even reaching the bridge.
"You know how it is," he said. "Some of us said some provocative things, and the tear gas started." The protesters threw rocks, and the confrontation quickly escalated, Mr. Saeed and others said. The Morsi supporters feared that the police were preparing to storm their encampment, so they started building brick walls on the road to "to prevent them from coming into the sit-in," Mr. Saeed said.
An hour and a half after the clashes started, the police and their allies started firing live ammunition and pellet guns, Mr. Saeed said. Other witnesses said they saw snipers on the roofs of nearby buildings.
Ahmed Hagag was there with his best friend, Ashraf, both of them woefully underequipped for a fight. "We went there with masks and vinegar," he said, in preparation for the tear gas. Ashraf, who had been "yearning for martyrdom," did not want to stand in the back, Mr. Hagag said. "So it happened, and a bullet ended up in his heart."
The violence went until morning, choking a field hospital nearby with bodies and patients near death.
Before the police retreated around 8 a.m., a spurt of gunfire came from their positions, sending people scrambling for cover and setting off a new stampede of ambulances.
In the morgue of the field hospital, 29 bodies sat in a row covered with white sheets. A medic, Mahmoud al-Arabi, said the wounds revealed a disturbing pattern of accuracy: many of the dead were shot in their head, chest or neck.
Other doctors walked around in a daze, near relatives in disbelief. They included a woman, who stumbled down the stairs after seeing a stricken relative.
"Do you need a mosque?" a volunteer asked, but the woman kept walking.
Later Saturday, the Health Ministry said 65 people had been killed. The Brotherhood said it had counted 66 dead and that an additional 61 people were "clinically dead."
The violence left the Brotherhood in an increasingly dire position, with limited options, said Samer S. Shehata, a professor of Arab politics at the University of Oklahoma and an authority on the Islamist group. "They really can't resort to violence -- they don't have a militia and it runs against all their rhetoric and recent history," Dr. Shehata said.
The group now faces the prospect of a broad legal ban of the kind it suffered under President Hosni Mubarak, and it is not clear that it could avoid that even if it agreed to stop the sit-ins and relinquish its demands for the reinstatement of Mr. Morsi.
The interior minister on Saturday raised the prospect of a new threat to the Brotherhood, saying he was reconstituting a state security agency that under Mr. Mubarak was responsible for monitoring Islamists and known for carrying out torture and forced disappearances. Without security agencies that had a political focus, Mr. Ibrahim said, "the security of the country doesn't work."
Robert F. Worth contributed reporting.world
This article originally appeared in The New York Times.