The Muslim Brotherhood-led National Alliance to Support Legitimacy (NASL) announced on Wednesday that it had documented 1,182 deaths in last August's violent dispersal of the Rabaa Al-Adaweya protest camp in support of ousted president Mohamed Morsi.
The pro-Morsi alliance announced its reported death toll at a press conference in Istanbul, Turkey, adding that another 500 were injured during the dispersal, with 350 reported missing.
The NASL statement comes in response to a previous estimate announced by the state-run National Council for Human Rights (NCHR) on Monday, which said that 632 people had been killed during the dispersal, including eight police officers, with 1,492 injured.
The NASL's last press conference in Cairo had been stormed by security forces and several of the attendees were reportedly arrested.
Clashes between Morsi supporters and security forces directly followed the Rabaa dispersal, causing many more deaths across the country.
Since then, security forces have been targeted in an escalating wave of attacks by militant groups, especially in the Sinai Peninsula.
Egypt's interim authorities designated the Brotherhood a terrorist organisation last December following the bombing of a security directorate in the Nile Delta city of Mansoura, despite repeated claims from the group condemning the violence and denying any responsibility.
Thousands of Brotherhood supporters, including the group's top leadership, have been arrested on a variety of charges, including inciting violence, espionage and terrorism.