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 on 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 had left the camp because it had become overcrowded, and that people had fanned out from the encampment along several boulevards. Others said they had planned to march through a nearby neighborhood.

The group that came under attack walked down Nasr Street, past the reviewing stand where 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 had 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 appeared to be a silver pistol and firing it, though it is not clear who the man was, or which side of the fighting he was on.

Mohamed Saeed, an agricultural engineer, said he and some of the other protesters had started to exchange words with the officers before even reaching the bridge.