An international human rights group said Tuesday that Egyptian authorities committed crimes against humanity due to the mass killings of anti-government protestors last August. 

Egyptian authorities killed hundreds of demonstrators -- most of whom supported Islamist President Mohamed Morsi and the Muslim Brotherhood -- during last August's anti-government protests. 

Egypt disputed the findings, which were released by Human Rights Watch, an international human rights group based in New York. Egypt prohibited senior experts at Human Rights Watch from entering the country to present the report, which was based on an investigation that was conducted over the course of a year. 

On Monday, Kenneth Roth, the executive director of Human Rights Watch, and Sarah Leah Whitson, director of the Middle East division, were deported from Egypt after being detained at the airport Sunday, according to The New York Times. 

It was the first time Egypt denied entry to employees of Human Rights Watch, according to Whitson. Airport officials put a form around Whitson's passport and checked off a box that said she was detained for "security reasons."

The group said an international commission of inquiry should be formed to investigate the "systematic" killings in the Rabaa Al-Adawiya Square in Cairo and other locations. 

At least 817 people, but possibly more than 1,000, were killed by security forces at Rabaa, in addition to many killed at other locales. 

The Human Rights Watch document urges that an independent investigation be launched into the role of President Abdel Fattah el-Sisi, then the military chief, and other senior military and security officials in the deaths. 

The document alleges that security officials were given free rein to use deadly force against the protesters, and that they had a plan to kill thousands of demonstrators to crack down on Morsi supporters. 

The protests and the killings occurred six weeks after the military removed Morsi from office. Morsi, Egypt's first democratically elected president, is now on trial for multiple capital offenses. 

The government in Egypt conducted an investigation into the deaths last year, when the pro-Morsi camps were disbanded by soldiers and police officers. A human rights panel, backed by the government, said the death toll was less than 700, and found that both officials and police officers used excessive force on protesters. 

Egypt's official State Information Service said Human Rights Watch's findings are influenced by "negativity and bias," and "ignored terrorist acts carried out by the Muslim Brotherhood and their supporters."

Egyptian authorities have implemented a harsh crackdown on the Muslim brotherhood, the region's biggest and oldest Islamist movement, in the past year. Egypt curbed the Brotherhood's basic rights, including freedom of speech and righ to assembly. In addition, Egyptian courts brought down mass death sentence verdicts against supporters of the Brotherhood. 

Government dissidents, both secular and Islamist, have been imprisoned under Egypt's severe anti-protest law. Activists as well as journalists, academics and filmmakers have been prosecuted for activities that the Egyptian government say endangers national security. Three journalists from Al Jazeera English, a broadcaster based in Qatar, were sentenced earlier in the summer to seven-year prison terms due to allegedly terroristic activities.