Egyptian security forces intentionally killed at least 817 protesters during last August's Rabaa massacre, in a premeditated attack equal to or worse than China's Tiananmen Square killings in 1989, Human Rights Watch (HRW) has argued in a report.

The 195-page investigation based on interviews with 122 survivors and witnesses has found Egypt's police and army "systematically and deliberately killed largely unarmed protesters on political grounds" in actions that "likely amounted to crimes against humanity".

The report recommends that several senior individuals within Egypt's security apparatus be investigated and, where appropriate, held to account for their role in the planning of both the Rabaa massacre and others that occurred last summer – including Abdel Fatah al-Sisi, Egypt's then defence minister and new president. As head of the army at the time, Sisi had overall responsibility for the army's role at Rabaa, and has publicly acknowledged spending "very many long days to discuss all the details".

The Rabaa massacre, which took place a year ago on Thursday, amid a week of violence from all sides, accompanied the dispersal of a six-week-old encampment in Cairo set up by demonstrators protesting the overthrow of President Mohamed Morsi. Egypt's military and police leaders maintain that the dispersal was carried out as humanely as possible, and only turned violent because an armed group within the camp started firing on security officials, killing eight policemen.

But HRW concluded that the camp's estimated 85,000 members were not given enough time to leave before troops started firing; were prevented for most of the day from leaving via safe exits; and used firearms "in only a few instances, which do not justify the grossly disproportionate and premeditated lethal attacks on overwhelmingly peaceful protesters".

The report documents scores of incidents where unarmed protesters were killed, often by snipers shooting from nearby buildings or, according to multiple witnesses, from helicopters flying overhead.

In similar testimony gathered by the Guardian, Hussein Abdel Aal, a 61-year-old former oil worker, said his son Ramzy had been shot dead by a sniper despite standing well back from the edge of the camp. "We were far from the frontline but my son got a bullet in his forehead, and it went out the back of his skull," said Abdel Aal, who was standing next to him. "He was shot by a sniper standing on top of the General Traffic Authority building – and he did not have any kind of weapons. From the stick to the gun, he had nothing."

Government officials say only 627 died at Rabaa, but HRW researchers counted at least 817, and suspect the real figure may actually top 1,000. Both figures exceed what HRW says are credible upper estimates of those who died at China's Tiananmen Square massacre.

Attempting to enter Egypt on Sunday to present the findings of the report, HRW's executive director, Ken Roth, and Middle East lead, Sarah Leah Whitson, were stopped and deported. The report's primary researcher, Omar Shakir, has since left the country, fearing arrest.

Egypt's interior and foreign ministries could not be reached for comment. But in a statement published on its Facebook page on Tuesday morning, the interior ministry, which runs Egypt's police force, condemned HRW as a group that worked in Egypt "above the law" and "without legal basis", and blamed its directors' deportation on their failure to procure the correct visas.

In a second statement emailed earlier to Egyptian reporters, an interior ministry spokesman stressed that the clearance was carried out with restraint, and followed weeks of negotiations to empty the sites peacefully, and several warnings on the day of the killings itself.

But survivors, interviewed by the Guardian at the time and in the months since, angrily disagree. Ahmed Husseini, a student who lost several friends at Rabaa, said: "The way they started the dispersal, it wasn't a way of ending a sit-in. It was a means of revenge. We had only ever seen people getting shot like this in a video game – I never thought I'd see it in reality."