At least 36 Islamist prisoners were killed on Sunday during an attempted jailbreak, in the latest violence since Egypt moved to crush supporters of ousted president Mohammed Morsi, AFP reported.
The violence, the deadliest in Egypt's recent history, comes amid a relentless security crackdown on Morsi's Muslim Brotherhood movement that includes the arrest of top Islamist leaders.
In Sunday's latest violence, gunmen ambushed trucks transferring 612 Islamist detainees to a north Cairo prison, and 36 inmates were killed in clashes that ensued when police returned fire, said the official MENA news agency.
The killings bring the death toll to almost 800 in five days of bloodshed sparked by a bloody police and military operation to clear Islamists from protest camps in Cairo.
In his first remarks since the campaign was launched, military chief Abdel Fattah al-Sisi on Sunday warned that security forces would confront any further violence from protesters.
"We will never be silent in the face of the destruction of the country," Sisi told top military and police commanders.
"We are very prepared for this," he said, pledging a "forceful" response to further attacks on police stations and government buildings.
Troops and police have sent reinforcements to the Abu Zaabal prison, the scene of Sunday's deadly jailbreak, MENA reported.
The Anti-Coup Alliance, which has been pressing for Morsi's reinstatement since the army toppled him in a popularly backed coup on July 3, accused police of killing 38 prisoners.
"They were reportedly assassinated in their truck with live ammunition and tear gas fired from windows," said the Brotherhood-led group, according to AFP.
The Islamists said they cancelled "several marches" on Sunday, citing fears of vigilantes and snipers, but that others would go ahead.
The Egyptian government announced on Saturday that it had begun deliberations on whether to ban the Muslim Brotherhood in the wake of the latest violence in the country.
The Brotherhood was outlawed for years until former President Hosni Mubarak was toppled in 2011, after which it swept to power in the country's first democratic elections.
Frustrated supporters of Morsi have been lashing out at Christian targets, setting fire to Coptic churches and other Christian centers in the country.
Egypt's Coptic Christian community makes up around 10% of the country's 90 million-strong population. Copts are indigenous to Egypt, their presence predating the Arab conquests of northern Africa.
Egyptian Copts have long complained of widespread discrimination, persecution and violence by both the Egyptian state and Islamist non-state actors. This has worsened in the days since Morsi’s removal.