Egypt’s military-backed government authorized security forces Thursday to fire live ammunition against opponents, underlining its determination to crush any lingering challenge posed by supporters of the country’s ousted president following a bloody crackdown on their camps.

A day after Egyptian soldiers and police killed hundreds of people in an assault on two Muslim Brotherhood protest camps set up to call for the reinstatement of deposed president Mohamed Morsi, the government pledged to use “all power” to confront the organization, creating the potential for further bloodshed.

With supporters of Morsi and of the military urging their followers to take to the streets again Friday, there seemed little prospect of an end anytime soon to the crisis that has paralyzed Egypt since June 30. That is when millions of protesters demanded the overthrow of their first democratically elected leader, prompting the military to detain Morsi and appoint a replacement.

There was further international fallout Thursday from the crackdown on the Brotherhood camps on the outskirts of Cairo, with President Obama announcing the cancellation of joint military exercises next month with Egypt. The U.N. Security Council, which met in an emergency session Thursday evening, issued a statement that urged an end to violence in Egypt and expressed sympathy “to the victims” but stopped short of blaming the government for the crackdown. Turkey withdrew its ambassador from Egypt to protest the shootings.

The nationwide civilian death toll rose Thursday to 578, according to Health Ministry spokesman Hamdi Abdo Wahid — 318 of them in Cairo and 260 in other parts of Egypt.

The largest single number of deaths — 288 — occurred in the vicinity of the Rabaa al-Adawiya mosque, which had become the epicenter of the protest movement. The mosque had housed most of the top Brotherhood leaders and members of Morsi’s government who escaped an initial dragnet when Morsi was toppled.

The Interior Ministry said Wednesday that 42 members of the security forces also were killed in the clashes.

In addition, 4,201 people were injured, and the death toll yet could rise from what is already the bloodiest single day since Egyptians rose up against the three-
decade-long presidency of Hosni Mubarak in January 2011.

Thursday’s tough statements from the government, coming on the heels of the ferocious crackdown, contributed to a sense that the military authorities have little interest in restoring Egypt’s brief experiment with democracy, said Shadi Hamid of the Brookings Doha Center in Qatar.

“This new government isn’t even pretending anymore that it is anything other than a full-on dictatorship,” he said. “Before yesterday’s violence there was at least a slight glimmer of hope for some kind of mediation or talks. Now we know there will not be an inclusive political process or a real democratic transition.”

While many of those killed were shot with bullets and witnesses have described seeing soldiers and police firing guns, the military had insisted that it had issued orders to its soldiers to use only tear gas and birdshot to control the demonstrations. The latest order represented an escalation in the level of approved force.

The Interior Ministry said the authorization to use live ammunition followed an attack Thursday in which government offices were set ablaze, allegedly by Brotherhood supporters, in the Cairo suburb of Giza, near the famous pyramids.

Citing the incident, the statement said that in order to “secure the homeland and to prevent attacks on lives and public and private property, the ministry has issued directions to all forces to use live ammunition in the face of any attacks on troops or installations.”

“All those tempted to try to tamper with the security of the homeland and its resources will be confronted firmly and decisively and in accordance with the law,” the statement added.

Egypt’s civilian cabinet said the month-long state of emergency imposed Wednesday, which gives the security services sweeping powers, is only a “temporary procedure forced by circumstances” and promised that some of its restrictions, which include a 7 p.m.-to-6 a.m. curfew, would be eased soon.The state of emergency imposed by Mubarak in 1981 lasted until after his overthrow in 2011.

Many Egyptians, who quickly became frustrated by the Brotherhood’s many failures in governing after the Islamist movement was elected last year, have welcomed the crackdown.

The Tamarod movement, which spearheaded the demonstrations to force Morsi’s departure, applauded the attacks on his supporters as an assertion of the values of the 2011 Egyptian revolution “against terrorism and the powers of darkness that want to drag us back centuries,” according to a statement broadcast on state television by Mohammed Abdel Aziz, one of the group’s leaders.

But the risk is high that previously peaceful protesters who survived Wednesday’s attacks will be driven to take up arms, Brookings’s Hamid said. Although full-scale civil war is unlikely, “you could have a real civil conflict that is still a very bad outcome for Egypt, and it means Egypt is not going to be able to recover economically or politically,” he said.

Scott Wilson in Washington and Colum Lynch at the United Nations contributed to this report.