Amnesty International has called for a full and impartial investigation into the bloodshed in Egypt, saying the authorities' response to the protests has been "grossly disproportionate".
The London-based human rights organisation also appealed for United Nations experts to be allowed to investigate the crackdown.
Backers of Egypt's ousted president Mohamed Morsi have pledged to stage daily demonstrations, as security forces besieged a Cairo mosque full of Islamist protesters after a day of clashes on Friday that left more than 80 dead.
Egypt is reeling from the deaths of 578 people on Wednesday when police cleared protest camps set up by loyalists of the deposed Islamist leader.
Amnesty claimed the security forces had used "unwarranted lethal force" and broken pledges to allow the wounded to exit safely, based on its research on the ground.
"Based on the initial testimonies and other evidence we've gathered, there seems to be little doubt the security forces have been acting with blatant disregard for human life, and full investigations that are both impartial and independent are urgently needed," said Philip Luther, Amnesty's Middle East and North Africa program director.
"While some protesters used violence, the authorities' response was grossly disproportionate, seemingly not differentiating between violent and non-violent protesters. Bystanders were also caught-up in the violence," he added.
Security forces used lethal force "when it was not strictly necessary," he said, adding that was a "clear violation of international law and standards".
"Previous promises to use graduated force when dispersing the sit-ins and provide ample warning and safe exits were quickly broken," the organisation said.
