More than 120 supporters of Egypt's ousted president, Mohamed Morsi, are feared dead after security forces moved in to clear the east Cairo site where thousands of protesters had been staging a month-long sit-in.
The exact toll remains unclear, with figures of dead and wounded diverging wildly after a night of running battles between protesters and security forces, in which more than 1,000 people were feared injured.
Doctors at a field hospital at the protest site, by the Rabaah al-Adawiyah Mosque in Nasr City, said 75 people had been killed and more than 1,000 injured.
The health ministry put the death toll at 20, with 177 wounded, with medics warning they could not cope with the number of casualties.
Many of the injured were hit by live fire, with Morsi supporters saying snipers had taken up positions on rooftops.
Muslim Brotherhood spokesman Gehad el-Haddad told Reuters: "They are not shooting to wound, they are shooting to kill."
Morsi was deposed in a military coup on 3 July after the army moved in to restore order following large-scale protests in Tahrir Square, scene of the Arab Spring uprising against former president Hosni Mubarak.
Hundreds of thousands of Egyptians had demonstrated in opposition to the religious agenda of Morsi's Muslim Brotherhood party, which won the election in June last year following the deposing of Mubarak after 30 years in charge.
Interior Minister Mohammed Ibrahim had pledged on Saturday to end the sit-in at the mosque, saying residents had complained about the encampment, and vowing it would be dealt with "soon and in a legal manner".
It remained unclear whether the violence had started in a concerted move by the army against protesters, or had been sparked by more localised clashes.
Conflicting reports suggested the violence broke out as supporters blocked a main road, and the army had moved to stop them.
Senior Brotherhood politician Saad el-Hosseini said security forces had moved in aggressively to clear the area.
"I have been trying to make the youth withdraw for five hours. I can't. They are saying they have paid with their blood and they do not want to retreat," he said.
State news agency Mena quoted an official saying security forces had been trying to stop fighting between rival sides, and that eight security personnel had been injured.
No live fire was used, only tear gas, the official said
Tens of thousands of Muslim Brotherhood supporters remain camped in the protest zone.
Supporters and opponents of Morsi had staged rival protests overnight across the capital, with anti-Morsi groups camped out in Tahrir Square and around the Itihadiya presidential palace.
Army chief Gen Abdel Fattah al-Sisi had urged people to take to the streets to legitimise the army's intervention.
Violence was also reported in Egypt's second city, Alexandria, where at least 10 people were killed in clashes between rival groups.
The Brotherhood's political arm, the Freedom and Justice Party, called for more protests in the Mediterranean port city.
Well over 200 people are known to have died in violence since the overthrow of Morsi, most of them Brotherhood supporters.
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