The executive director of Human Rights Watch and another senior staff member have been denied entry to Egypt.
Kenneth Roth and Sarah Leah Whitson were in Cairo to launch a report by the group on mass killings last year.
Hundreds of people were killed on 14 August 2013 after security forces broke up protests against the ousting of President Mohammed Morsi.
President Abdul Fattah al-Sisi, who was elected head of state in May, was the army chief at the time of the killings.
Human Rights Watch (HRW) is one of a number of rights groups that have expressed alarm at a broad crackdown on dissent by authorities since the army's overthrow of Mr Morsi in July 2013.
After arriving at Cairo International Airport, the pair were detained by officials for 12 hours before being put on planes out of the country.
Ms Whitson, the head of HRW's Middle East and North Africa division, said Egyptian officials told her they were being deported for "security reasons".
In a statement, HRW said it was the first time that Egyptian authorities had denied its staff members entry to the country, including during the rule of ex-President Hosni Mubarak.
The group said its report, due to be released on Tuesday, "documents how Egyptian police and army methodically opened fire with live ammunition on crowds of demonstrators" after Mr Morsi's removal from power.
At least 1,150 people were killed at six demonstrations in July and August 2013, according to HRW. The worst incidents were at Rabaa al-Adawiya mosque and Nahda Square in Cairo.
Tweeting from Cairo's airport, Mr Roth said the "Rabaa massacre numbers rank with Tiananmen and Andijan" but the Egyptian government "wouldn't let me in to present a report on it".
He said the group had shared its findings with President Sisi's government and had hoped to discuss the report with the authorities.
The government is yet to comment on the incident.