Supporters of Egypt's Muslim Brotherhood stormed and torched a government building in Cairo on Thursday, while families tried to identify hundreds of mutilated bodies piled in a Cairo mosque a day after they were shot dead by the security forces.
Egypt's health ministry says 623 people were killed and thousands wounded in the worst day of civil violence in the modern history of the most populous Arab state.
Brotherhood supporters say the death toll is far higher, with hundreds of bodies as yet uncounted by the authorities, whose troops and police crushed protests seeking the return of deposed President Mohamed Morsi.
State television quoted the Interior Ministry as saying the security forces would again use live ammunition to counter any attacks against themselves or public buildings.
The UN Security Council will meet later on Thrsday to discuss the situation after a meeting was requested by council members France, Britain and Australia.
International condemnation rained down on Cairo's military-backed rulers for ordering the storming of pro-Morsi protest camps after dawn on Wednesday, six weeks after the army overthrew the country's first freely elected leader.
The US State Department said it would review aid to Egypt "in all forms" after US President Barack Obama cancelled plans for upcoming military exercises with the Egyptian army, which Washington funds with $1.3 billion in annual aid.
"The United States strongly condemns the steps that have been taken by Egypt's interim government and security forces," Obama said in an address from his vacation home on the Massachusetts island of Martha's Vineyard.
"We deplore violence against civilians. We support universal rights essential to human dignity, including the right to peaceful protest."
Obama's Defense Secretary Chuck Hagel later warned Egypt's army chief that "the violence and inadequate steps towards reconciliation are putting important elements of our longstanding defense cooperation at risk".
Western diplomats have told Reuters that senior US and European officials had been in contact with Egypt's rulers until the final hour, pleading with them not to order a military crackdown on the protest camps, where thousands of Morsi's followers had been camped out since before he was toppled.
There were reports of protests on Thursday but no repeat of the previous day's bloodbath. In Alexandria, Egypt's second largest city, hundreds marched, chanting: "We will come back again for the sake of our martyrs!"
Brotherhood spokesman Gehad El-Haddad said anger within the 85-year-old Islamist movement, which has millions of supporters across Egypt, was "beyond control".
"After the blows and arrests and killings that we are facing, emotions are too high to be guided by anyone," he said.
The Brotherhood has called on followers to march in Cairo later on Thursday, while funeral processions for those who died provide further potential flashpoints over the coming days.
In Cairo, Reuters counted 228 bodies, most of them wrapped in white shrouds, arranged in rows on the floor of the Al-Imam mosque in northeast Cairo, close to the worst of the violence.
The mosque had been converted into a charnel house, resembling the aftermath of a World War One battlefield. Medics pushed burning incense sticks into blocks of ice covering the bodies and sprayed air freshener to cover up the stench.
Some men pulled back the shrouds to reveal badly charred corpses with smashed skulls. Women knelt and wept beside one body. Two men embraced each other and shed tears by another.
The bodies, piled there because morgues and hospitals were full, did not appear to be part of the official tally of 525 killed, which also includes more than 40 police and hundreds killed in clashes outside of the capital.
Several thousand people gathered in the square outside the mosque, chanting: "The army and the police are a dirty hand!"
In the Giza section of Cairo, Morsi supporters set fire to a governorate building, while state television said two police officers were killed in an armed attack on a police checkpoint in the area.