The Muslim Brotherhood said at least 31 people were killed on Saturday when security forces opened fire on a protest by supporters of deposed President Mohamed Mursi in Cairo.
"They are not shooting to wound, they are shooting to kill," Muslim Brotherhood spokesman Gehad El-Haddad said. "The bullet wounds are in the head and chest."
The violence erupted on the fringes of a round-the-clock vigil being staged by backers of Mursi, who was ousted from power earlier this month by Egypt's military following mass protests against his first year in office.
Al Jazeera's Egypt television station showed medics desperately trying to revive casualties arriving at a field hospital at the Brotherhood sit-in at Rabaa al-Adawiya, a mosque in northeast Cairo.
El-Haddad said police started firing repeated rounds of tear-gas at protesters on a road close to the mosque sometime after 3.00 am (0100 GMT). Shortly afterwards, live rounds started flying, hitting people at close range.
The bloodshed came the day after supporters and opponents of Mursi staged mass rival rallies across the country, bringing hundreds of thousands into the streets and laying bare deep divisions within the Arab world's most populous country.
Well over 200 people have died in violence since the overthrow of Mursi, most of them Brotherhood supporters.
There was no immediate word from the security forces about what they thought had happened at Rabaa early Saturday.