CAIRO - Egypt's Muslim Brotherhood called on followers to march in protest in Cairo on Thursday, after at least 525 people were killed in a security crackdown on the Islamist movement that has left the most populous Arab nation polarised and in turmoil.

A Reuters witness counted 228 bodies, most of them wrapped in white shrouds, arranged in rows on the floor of the Imam mosque in northeast Cairo, close to the worst of Wednesday's violence between police and demonstrators.

Islamist supporters of former President Mohamed Morsi, ousted by the army on July 3, clashed with police and troops who used bulldozers, teargas and live ammunition to clear two Cairo sit-ins that had become a hub of resistance to the military.

The clashes spread quickly, and a health ministry official said on Thursday that 525 people were killed and more than 3,500 injured in fighting in Cairo, Alexandria and numerous towns and cities around the mostly Muslim nation of 84 million.

The capital and other areas hit by shocking violence were largely calm overnight, after the army-installed government declared a month-long state of emergency and imposed a dusk-to-dawn curfew on the capital and 10 other provinces.

But whether the powerful military can keep a lid on the fury felt by millions of Morsi's supporters, most of them from his Muslim Brotherhood movement, is unclear.

The next potential flashpoint comes later on Thursday, after Morsi's Brotherhood movement called for marches in the capital to protest the deaths. In Alexandria, Egypt's second biggest city, protesters were already on the move by early afternoon.

A military source said that while sit-ins like the main one outside the Rabaa al-Adawiya mosque in Cairo would no longer be tolerated, marches may be in spite of the state of emergency.

Funerals of those killed will also be held throughout the day, and on Friday, the main Muslim day of prayer, anger could spill on to the streets.

"I think tomorrow will be a big day for protests throughout Egypt, with the potential for violence very high," said Yasser El-Shimy, Egypt analyst with the International Crisis Group.