Human Rights Watch's executive director Kenneth Roth and Middle East and North Africa director Sarah Leah Whitson had flown to Cairo to launch a report on the mass killings of protesters by security forces a year ago, weeks after the army removed elected Islamist President Mohamed Mursi from power.
Human Rights Watch is one of a number of international rights groups that have expressed their alarm over an increasingly broad crackdown on dissent by authorities since then-army chief Abdel Fattah al-Sisi seized power in July last year.
The report comes after a year-long investigation, including interviews with more than 200 witnesses to the killings in July and August of 2013.
It details the conduct of government security forces confronting protesters demanding Mr Mursi's reinstatement.
Mr Roth said: "We came to Egypt to release a serious report on a serious subject that deserves serious attention from the Egyptian government. Instead of denying the messenger entry to Egypt, the authorities should seriously consider our conclusions."