Egypt's military-backed government on Sunday banned vigilante groups and issued a fresh warning that it would not tolerate violence from civilians, as it struggled to prevent a crisis over the country’s deposed Islamist leader from spinning further out of control.

Civilian “people’s committees” armed with clubs, sticks and guns have appeared on Egyptian streets in recent days, in an ominous turn to the bloody saga that has unfolded since President Mohamed Morsi was ousted on July 3. More than 800 civilians and 70 members of the security forces have been killed in violence since a crackdown on Morsi supporters began Wednesday. The state-run Middle East News Agency reported that 79 of the deaths occurred Saturday.

Even as authorities seemed to be trying to tamp down the violence, though, an alliance representing Morsi’s supporters said Sunday that 52 detainees rounded up during the recent demonstrations were killed by police while they were being transferred to a prison in Qalyubia, north of Cairo. The government put the number of deaths in the incident at 36.

And in a sign of the simmering tensions between the government and the country’s largest Islamist group, the Muslim Brotherhood, thousands of demonstrators calling for Morsi’s reinstatement took to the streets Sunday for the third day in a row.

Security forces, however, closed roads and deployed tanks to choke off protests in Cairo, blunting the usually impressive ability of the Brotherhood to turn out its supporters. “The army is everywhere. Just walking on the streets earlier today you could see their increased presence,” said Abdelrahman Mohammed, a volunteer with the anti-coup media campaign, explaining why the group called off one major demonstration.

Egypt’s defense minister and de facto leader, Gen. Abdel Fatah al-Sissi, in a televised speech Sunday to an audience of uniformed military and security officials, vowed to deal forcefully with perpetrators of violence.

Sissi, who led the coup that toppled Morsi, also said that supporters of the ousted regime would be welcome to participate in politics in the future, but it was unclear whether that offer extended to members of the Brotherhood, which is allied with Morsi. The interim government installed by Sissi has hinted that it is considering instituting a ban on the Brotherhood.

“We will not be silent in the face of the destruction of the country and the people, of the burning of the homeland and terrorizing innocent people,” Sissi said, adding that Morsi supporters are welcome “to participate in rebuilding of the democratic path and to engage in the political process, according to the map of the future rather than confrontation and destruction of the Egyptian state.”

But an alliance of Morsi supporters said in a statement Sunday that Sissi’s conciliatory comments on political inclusion were belied by the slaying of 52 detainees, which “confirms the systematic violence practiced against opponents of the coup.”

Egypt’s Interior Ministry said 36 prisoners died in the incident, in which it said 612 members of the Brotherhood had tried to escape, in the process taking a police officer hostage. The prisoners died of suffocation when tear gas was fired or during a stampede by some trying to get away, the ministry reported. Security forces rescued the police officer, the ministry said.

The military broke up two Brotherhood protest camps in Cairo last week, killing hundreds of demonstrators who had occupied the sites to demand Morsi’s reinstatement. The interim government has imposed a dusk-to-dawn curfew and declared a state of emergency to try to restore order and curtail protests over Morsi’s removal.

Signaling fears that violence could escalate, though, the Interior Ministry on Sunday outlawed “people’s committees,” saying they had been “exploited by others in conducting acts against the law.” The pro-army vigilante groups have participated in efforts to quell the protests, in a sign of the widespread sense of fury at a president who won a historic democratic election a year ago but lost support as the economy crumbled and crime soared.

Some participants in pro-Morsi demonstrations, however, have also brandished weapons, despite a call from the Brotherhood to keep the protests peaceful.

“The military doesn’t want to lose control of popular sentiment. They want to be the ones calling the shots” — even though they may have benefited from some of the armed groups’ actions, said Shadi Hamid, director of research at the Brookings Doha Center in Qatar.

An increasing number of U.S. lawmakers are calling for a suspension of Washington’s $1.5 billion in annual military aid to Egypt because of the crackdown on Morsi supporters. On Sunday, Foreign Minister Nabil Fahmy responded by saying that foreign pressure would “only lead to more polarization and tension.”

“I asked the competent departments in the Ministry of Foreign Affairs to review the kind of aid we receive from other countries and whether that aid is used in the optimal way,” he said.

Fahmy also said that a fact-finding committee was going to investigate “everything that has taken place after the revolution of the 30th of June,” referring to the widespread protests that led to Morsi’s removal by the military.

The military continued to detain Muslim Brotherhood officials on Sunday, according to the state news agency. It said that 17 Brotherhood leaders were arrested in the cities of Gharbiya and Behira.

Abigail Hauslohner, Lara El Gibaly and Amer Shakhatreh contributed to this report.