Egypt plunged further into civil conflict Friday after anti-government demonstrations led by the Muslim Brotherhood erupted in violence involving the security forces, protesters and armed civilians on both sides of the political divide.

Gunfire crackled in the capital, armored vehicles sped through the streets, civilians carrying pistols and machetes set up checkpoints, and smoke billowed through the sky as what was billed by the Brotherhood as a Day of Rage turned into a frightening glimpse of what might lie ahead for the deeply polarized country.

Adding to concerns about a spiral into chaos was the absence of calls for restraint or calm from any of the parties to the conflict. Both the Brotherhood and the military-backed government seemed to be digging in for a long fight, with little sign that either was prepared to compromise.

The Muslim Brotherhood claimed that 213 people were killed on a day that brought the violence to the heart of the capital and drew in for the first time government-approved Popular Committees, whose members set up checkpoints carrying sticks, machetes and, in some instances, guns. The government declined to issue casualty figures.

The number of dead appeared to be lower than the more than 600 people killed in Egypt on Wednesday. But the Brotherhood put the toll at more than 100 in central Cairo alone, after security forces used live ammunition to suppress demonstrators who had gathered at Ramses Square in what the group called “glorious heroic scenes.”

Protester camps in Cairo

Violence was also reported in several other cities across Egypt, including Alexandria and Ismailiya, where a video posted on YouTube showed troops opening fire on an unarmed demonstrator who stood in front of a tank. The Brotherhood called for protests to continue for a week longer, to show opposition to last month’s coup and this week’s brutal crackdown on civilian protest camps that ignited the latest wave of violence.

But the government, appointed by the military after the coup, showed no sign of wavering. In a statement on its Facebook page, the cabinet declared that “the Egyptian armed forces, the police and the great people of Egypt are standing as one hand, in the face of the brutal terrorist plot by the Muslim Brotherhood on Egypt.”

The language underscored the extent to which Egyptians who rallied behind calls for the overthrow of President Hosni Mubarak in 2011 have turned against one another in the intervening years, culminating in the demonstrations and the coup that deposed the elected government of President Mohamed Morsi on July 3.

The White House had no further comment on the violence, but Susan E. Rice, the national security adviser, briefed President Obama on the situation before he went on a bike ride with his family on Martha’s Vineyard, where he is vacationing. Obama on Thursday announced the cancellation of joint military exercises with Egypt scheduled for next month, but he left $1.3 billion in annual aid to the country untouched.

Two leading Republicans who recently visited Egypt condemned the actions of the interim government. “The massacre of civilians this week in Egypt has brought our long-standing relationship with that country to a fork in the road,” Sens. John McCain (Ariz.) and Lindsey O. Graham (S.C.) said in a joint statement. “The interim civilian government and security forces — backed up, unfortunately, by the military — are taking Egypt down a dark path, one that the United States cannot and should not travel with them.”

The shooting erupted early in the afternoon, apparently after security forces opened fire on Morsi supporters streaming into Cairo’s central Ramses Square from around the city after Friday prayers, although it was unclear who opened fire first or why.

Scores of dead and wounded were carried into a field hospital set up by Morsi supporters at the al-Fateh mosque in Ramses Square on Friday afternoon, and by nightfall more than 64 bodies had been laid out inside, according to Hani Nawari, one of the doctors treating the injured. Late Friday night, as helicopters circled repeatedly over the area, the Muslim Brotherhood issued an appeal for help, saying that thousands of people were trapped inside by the army under “continuous shooting.”

Clashes erupted elsewhere as Morsi supporters attempting to reach the square were blocked by pro-military vigilantes, who had formed “popular committees’’ in response to a call by activist group Tamarod, which led the protests that resulted in Morsi’s ouster.

One pitched battle along the 15th of May bridge spanning the Nile raged throughout the afternoon as vigilantes and police sought to prevent protesters from crossing to join the Ramses demonstration. On at least a half-
dozen occasions, the pro-military forces opened fire with live ammunition to send their opponents scurrying backward on the bridge. Molotov cocktails flew through the air and the gunfire rattled across the Nile, sending protesters and ordinary civilians fleeing down side streets and into buildings.

At a checkpoint nearby in the upscale Nile island neighborhood of Zamalek, a member of one pro-military vigilante committee said demonstrators on the other side of the clash had also been armed.

“They were shooting us, so we were shooting them,” he said as his colleagues, armed with metal sticks, stopped cars, checked identification cards and searched trunks. “They’re just terrorists,” he said of the protesters. “We are fighting terrorists, not Muslims.”

The Brotherhood insists that its demonstrators are not armed, but there were several reports of armed men seen among the protesters. State television showed a masked man firing an assault rifle from a bridge and moving among pro-Morsi protesters, although his identity could not be confirmed.

It was nevertheless clear that the vast majority of the people who had gathered in Ramses Square as the shooting erupted were unarmed, and many were women and children. “The people want the toppling of the regime,” they chanted as they converged on the square, echoing the call that had defined the original revolution more than two years ago.

As night fell and the 7 p.m. curfew imposed earlier in the week went into effect, the streets emptied and the gunfire subsided, except around the square. But the Popular Committee checkpoints remained in place, their members apparently exempt from the order to stay indoors.

Sharaf al-Hourani and Amer Shakhatreh contributed to this report.