NEW YORK - The systematic and widespread killing of at least 1,150 demonstrators by Egyptian security forces in July and August 2013 probably amounts to crimes against humanity, a prominent international human rights watchdog body said Tuesday.

In a report based on a year-long investigation, New York-based Human Rights watch said the dispersal of the Rab’a al-Adawiya sit-in on Aug. 14 alone, security forces, following a plan that envisioned several thousand deaths, killed a minimum of 817 people and more likely at least 1,000.

The 188-page report says, ‘All According to Plan: The Rab’a Massacre and Mass Killings of Protesters in Egypt,’ documents the way the Egyptian police and army methodically opened fire with live ammunition on crowds of demonstrators opposed to the military’s July 3 ouster of Mohamed Morsi, Egypt’s first elected civilian president, at six demonstrations between July 5 and August 17, 2013. While there is also evidence that some protesters used firearms during several of these demonstrations, Human Rights Watch was able to confirm their use in only a few instances, which do not justify the grossly disproportionate and premeditated lethal attacks on overwhelmingly peaceful protesters.

‘In Rab'a Square, Egyptian security forces carried out one of the world’s largest killings of demonstrators in a single day in recent history,’ Kenneth Roth, executive director of Human Rights Watch, said in a statement. ‘This wasn’t merely a case of excessive force or poor training. It was a violent crackdown planned at the highest levels of the Egyptian government. Many of the same officials are still in power in Egypt, and have a lot to answer for.’

The authorities have failed to hold even a single low-level police or army officer accountable for any of the killings, much less any official responsible for ordering them, and continue to brutally suppress dissent. In light of the continued impunity, an international investigation and prosecutions of those implicated are needed, Human Rights Watch said. States should further suspend military and law enforcement aid to Egypt until it adopts measures to end its serious rights violations.

Human Rights Watch interviewed more than 200 witnesses, including protesters, doctors, local residents, and independent journalists, visited each of the protest sites during or immediately after the attacks began, and reviewed physical evidence, hours of video footage, and statements by public officials, it said. Human Rights Watch said it wrote to relevant Egyptian ministries soliciting the government’s perspective on these events, but received no responses.

The report includes a detailed examination of the planning and execution of the dispersal of the protest at Rab’a al-Adawiya Square, where from July 3 to August 14, tens of thousands of largely peaceful Morsi supporters, including women and children, held an open-ended sit-in demanding Morsy’s reinstatement. Human Rights Watch used satellite photographs from one night of the sit-in, August 2, to estimate that approximately 85,000 demonstrators were in the square that night. ‘Evidence showing how security forces opened fire on crowds of protesters from the first minutes of the dispersal belies any claims that the government sought to minimize casualties,’ Roth said. ‘The brutal way security forces dispersed this demonstration resulted in a shocking death toll that anyone could have foreseen, and indeed the government had anticipated.’