Human Rights Watch (HRW) said Egyptian President Abdel Fattah el-Sissi should be investigated for possible human rights violations in connection with the killing of hundreds of protesters last year in Cairo.
In a new report Tuesday, the group outlines the conclusions of a yearlong investigation into six weeks of violent crackdowns on protesters who were rallying against the ouster of former leader Mohamed Morsi.
More than 1,000 people were killed in what HRW called an "unprecedented scale" of protester deaths in Egypt as the country's police and security forces "systematically and intentionally used excessive force."
The report said Sissi, who was serving as Egypt's army chief, should be investigated along with Interior Minister Mohamed Ibrahim and Medhat Menshawy, who led forces that carried out a massively deadly operation to clear out a protest camp.
The government had warned for days it would move against the camp around the Rabaa al-Adawiya mosque, which officials said was disruptive and could incite terrorism.
But the report said warnings about when the action would take place were not sufficient.
HRW said there was some evidence of protesters attacking security forces at the site, but that the response "amounted to collective punishment of the overwhelming majority of peaceful protesters."
More than 800 people were killed.
The group said the pattern of responses to the pro-Morsi protests between July 5 and August 17 last year amounted to "grossly disproportionate and premeditated lethal attacks," and that the killings likely amounted to crimes against humanity.
The report said Egypt has not carried out any credible judicial investigations or prosecutions and urged the government to probe those responsible for any rights violations.
It also recommended security forces stop "unlawful excessive use of force," and for officials to cooperate with Egypt's own fact-finding commission related to the mass killings.
Human Rights Watch also urged the United Nations to establish a commission of inquiry to investigate all human rights violations from the mass killings of protesters in Egypt.
Sissi led the ouster of Morsi, who was Egypt's first democratically elected president but lasted only a year in office before protesters held mass rallies accusing him of trying to monopolize power and failing to fix the country's economy.
The crackdown against Morsi's Muslim Brotherhood also included arresting many of the group's top leaders.
Ahead of the report's release, two Human Rights Watch officials said Egypt barred them from entering the country.
Executive director Kenneth Roth and Middle East director Sarah Leah Whitson said Monday that authorities held them overnight at the Cairo airport before denying them entry for "security reasons."
State Department spokeswoman Marie Harf said the United States was disappointed the two individuals were not allowed to enter Egypt and that the U.S. encourages the Egyptian government to conduct a transparent investigation of the protester deaths.