Amnesty International said on Wednesday it had evidence pointing to the "disproportionate" use of lethal force by Egyptian security forces, and called for them to be reined in to avoid "disaster".
The claim by the human rights watchdog comes two days after at least 51 people, most of them supporters of the ousted Islamist president Mohamed Morsi, died in clashes outside the Cairo headquarters of the elite Republican Guard.
Morsi's Muslim Brotherhood claimed police and troops "massacred" their supporters as they performed dawn prayers, while the army said it came under attack by "terrorists."
"Despite claims by the military that protesters attacked first during clashes on Monday and that no women and children were injured, first hand accounts collected by Amnesty International paint a very different picture," the group's deputy regional director Hassiba Hadj Sahraoui said in a statement.
"Even if some protesters used violence, the response was disproportionate and led to the loss of life and injury among peaceful protesters," she added.
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The rights watchdog said visits to morgues, hospitals and sites of violence in Cairo and Alexandria, and testimonies gathered from injured protesters and relatives of victims suggested "the use of disproportionate force by the security forces, including intentional lethal force".
"Many of those killed and injured had been shot in the head and upper body with shotgun pellets and live ammunition," Amnesty added.
It said that unless the security forces were reined in and clear orders given on the use of force, "we're looking at a recipe for disaster".
The military ouster of Egypt's first democratically elected president last week, after massive protests calling for his resignation, has pushed the divided country into a vortex of violence.
Amnesty said at least 88 people have died in protests and political violence since Friday, including three members of the security forces, with some 1,500 people wounded in the unrest.