The New-York based Human Rights Watch called Tuesday for an international commission of inquiry into mass killings in Egypt last summer that left hundreds dead, saying they likely amount to crimes against humanity.

Based on a year-long investigation into the incidents that followed the ouster of Islamist President Mohammed Morsi on July 3, 2013, Human Rights Watch called specifically for an inquiry in to the role of country's current President Abdel-Fattah el-Sissi, and at least 10 senior military and security chiefs in the killing of 1,150 protesters in the span of six weeks.

The group said it found that authorities had used excessive and deliberate force against protesters on political grounds in successive attacks on their gatherings.

The worst incident of mass killings occurred on Aug. 14, when authorities opened fire on a massive pro-Morsi sit-in at Cairo's Rabaah al-Adawiyah square, leaving at least 817 dead after 12 hours, Human Rights Watch said. The group called it the "world's largest killings of demonstrators in a single day in recent history."

Rights chiefs barred from entering Egypt

Two executives from Human Rights Watch, arriving from New York on Sunday, were barred from entering Egypt and were turned back ahead of a planned launch of the report in Cairo. Officials said they were barred from entering after the government asked them to postpone their visit. The executives refused and insisted on coming, the Interior Ministry said Tuesday.

The report's authors said they had been in touch with government officials throughout their investigation, asking them repeated questions about their policies and planning, but received no response.

Egypt Muslim Brotherhood

A supporter of ousted Egyptian President Mohammed Morsi carries wood to burn in a fire barricade at the sit-in at Rabaa Al-Adawiya Square in Cairo's Nasr City district, Egypt on Aug. 14, 2013. (Manu Brabo/Associated Press)

One of the main researchers of the HRW report also left the country after the executives were barred from entering.

The report's findings provide a detailed look at the government's policies and measures against successive protests by pro-Morsi supporters in the days following his ouster.

An Interior Ministry official had no immediate comment on the report when asked about it. Instead, he said a local quasi-governmental rights group had carried out its own investigation. He spoke on condition of anonymity because he was not authorized to talk to media.

Human Right Watch accused authorities of using "deliberate and indiscriminate lethal force to disperse the two sit-ins, where protesters had remained encamped for 45 days, resulting in one of the most bloody incidents against protesters in recent history.

Gunmen shot from helicopters: witnesses

While Egyptian security forces have often used excessive force to respond to demonstrations, the August 14 dispersals were unprecedented in the scale of sheer brutality." The group said the government failed to identify a specific time for the dispersal or give sufficient warnings for the protesters to leave the area.

The report found that the attack on the encampment at Rabaah was carried from five different directions, with witnesses saying there were gunmen shooting down from helicopters at protesters who were being besieged with no access to safe exits for most of the day. Snipers took positions on rooftops overlooking the sit-in and fired from there, it said.

Egypt Muslim Brotherhood

Wounded supporters of ousted Islamist President Mohammed Morsi lie on the floor of a makeshift hospital at a sit-in at Cairo's Nasr City district, Egypt. The scene on Aug. 14, 2013, was the start of the biggest massacre in modern Egyptian history. (Manu Brabo/Associated Press)

The report also stated that "the brutal manner in which the security forces carried out the Rabaah and al-Nahda dispersals appears to reflect policies that the Egyptian authorities at the highest levels implemented after weeks of planning," the report said.

The New York based-group said that after its yearlong investigation, it concluded that the Egyptian government used "disproportionate force, failed to take measures to minimize loss of life and knowingly opened fire on unarmed protesters."

"The systematic and widespread nature of the deliberate and indiscriminate killings, coupled with evidence indicating that the government anticipated and planned to engage in mass unlawful killings ... indicate that the violations likely amount to crimes against humanity," it added.

'Systematic campaign'

The government had said the sit-ins had constituted a disruption of public order and security and accused the encampments of harbouring "terrorists."

However, the Human Rights Watch report said the dispersals of the large sit-in were not isolated developments, but rather "part of a systematic campaign by the Egyptian government to violently disperse dissent."

The government's final toll for the Rabaah killings was 624 dead. Morsi's supporters say they documented names of 2,500 dead, though the highest tallies by independent rights groups have been close to 1,000. At least eight policemen were killed during the dispersals.

Soon after Morsi's ouster, authorities repeatedly used lethal force to disperse protests. In four different incidents investigated by HRW in July and August before the dispersal of the sit-ins, at least 281 protesters were killed when security and military opened fire on the crowd.

The group suggested that "some protesters" carried weapons and shot at police. But based on interviews with 200 survivors, witnesses, including protesters, local residents, medics and journalists, "they were few in numbers" and didn't justify the indiscriminate firing at unarmed protesters, the group said.

The report said that rather than investigating potential wrongdoing, the government has refused to acknowledge any possible infractions on the part of the security forces. No formal investigation by prosecutors has been made public and no single officer or any official has been held accountable.

Instead, the government has accused foreign correspondents of biased coverage, and provided them with material, footage and photos, to show that the encampments were training grounds for militants, often failing to authenticate the material or provide details on when it was taken or where, the report said.