Thirteen Egyptian and international human rights organisations on Tuesday urged Cairo's military-installed authorities to probe the mass killing of Islamist protesters in the capital on August 14.
The joint call issued by organisations that included Amnesty International, Human Rights Watch and Egyptian Initiative for Personal Rights, said an investigation must be launched into the killing of "up to 1,000 people by security forces" almost four months ago when they dispersed sit-ins by supporters of deposed president Mohamed Morsi.
"There can be no hope for the rule of law and political stability in Egypt, much less some modicum of justice for victims, without accountability for what may be the single biggest incident of mass killing in Egypt's recent history," said Gasser Abdel-Razak, associate director at the Egyptian Initiative for Personal Rights.
Egyptian police moved in to disperse the two protest camps after Morsi supporters refused to leave despite repeated warnings. Hundreds of people, mainly Morsi supporters, were killed in the ensuing clashes.
The incident was the worst mass killing in Egypt's modern history and deeply polarised the country less than three years after longtime autocrat Hosni Mubarak was toppled by pro-democracy protests.
Thousands of supporters of Morsi, mostly Islamists from his Muslim Brotherhood movement, have been arrested since the August crackdown.
Morsi, Egypt's first democratically elected leader, was forced from power by the military on July 3 amid massive protests against his turbulent year-long rule.
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He has since been put on trial along with several aides and top leaders of the Brotherhood, while the movement's activities have been banned.
"As a first step toward accountability, the government should establish an effective independent fact-finding committee to investigate responsibility throughout the chain of command for the unlawful killings," the rights groups said.
They said that on August 14 a "small minority of protesters used firearms... but the police responded excessively by shooting recklessly, going far beyond what is permitted under international law."
"After the unprecedented levels of violence and casualties seen since the ousting of Mohamed Morsi, investigations must provide real answers and cannot be another whitewash of the security forces' record," Hassiba Hadj Sahraoui of Amnesty International said in the statement.
"Egypt's authorities cannot deal with the carnage through PR in world's capitals, rewriting events and locking up Morsi's supporters."
The groups also said the probe should determine whether there is any evidence of a policy to kill protesters or commit other serious crimes.
The groups said investigations must also be launched into a series of sexual assaults that took place in Cairo's Tahrir Square -- epicentre of the 2011 anti-Mubarak revolt -- which successive governments have ignored over the past three years.
It said between June 28 and July 7 of this year, when the square was the epicentre of protests calling for Morsi's ouster, mobs of men sexually assaulted and in some cases raped at least 186 women.