Egypt's Muslim Brotherhood, fighting for its political survival, has accused security forces of killing dozens of detained Islamists, upping the pressure in a crisis that has rocked the Arab world's most populous state.
At least 850 people have died since last Wednesday in clashes pitting followers of deposed Islamist President Mohamed Mursi against the army-backed government in the worst bloodletting in Egypt's modern history.
As Western anxiety grows, Egypt's army chief vowed to stand firm in the face of violence on Sunday, calling on the Brotherhood to bow to the will of the people and accept the July 3 removal of Mursi, which followed mammoth street protests.
Senior European Union diplomats will meet in Brussels on Monday to review how best to leverage some 5 billion euros ($6.7 billion) of promised grants and loans, looking to apply pressure on Cairo's new rulers to find a compromise deal.
Since the fall of the autocratic Hosni Mubarak more than two years ago, Egypt has staggered from one crisis to another, alarming allies in the West and in neighboring Israel, with which it has had a peace treaty since 1979.
But Saudi Arabia warned the United States and Europe on Sunday against pressing the government to halt a crackdown on the Brotherhood, which it has always eyed with distrust.
The Interior Ministry said 36 Brotherhood members died during an attempted prison breakout on Sunday near the capital, saying the prisoners had been suffocated by tear gas.
Offering a very different version of events, a legal source told Reuters 38 men had died from asphyxiation in the back of a crammed police van.
The Brotherhood, battling to reverse the overthrow of Mursi, held the authorities responsible.
(Read more: Muslim brotherhood risks elimination)
"The murders show the violations and abuses that political detainees who oppose the July 3 coup get subjected to," it said.
Hundreds of Brotherhood supporters have been arrested in recent days across Egypt in an effort to end weeks of protests, but the group has said it will not retreat and staged rallies in both Cairo and Alexandria on Sunday.
The government says at least 70 members of the security forces have died in the past five days and the prime minister has proposed disbanding the 85-year-old Brotherhood.