Liz Throssell, a spokeswoman for the U.N. High Commissioner for Human Rights, said there was a general sense of outrage over reports of the slaying of 25 Egyptian police officers in the troubled Sinai Peninsula.
She expressed further concern over reports 36 prisoners died while in police custody Sunday. Equally disturbing were reports of mass arrests of senior members of the Muslim Brotherhood.
Ousted President Mohamed Morsi, a former high-ranking figure from the Brotherhood, was arrested following his July 3 military ouster. Mohamed Badie, 70, the spiritual leader of the Islamist movement, was arrested Tuesday in Cairo during a crackdown on the group.
"Everyone deprived of their liberty must be treated humanely and afforded all judicial guarantees under international law," Throssell said Tuesday.
Hundreds of Egyptians were killed last week in clashes between Brotherhood supporters and government forces.
Irina Bokova, director of the U.N. Scientific, Educational and Cultural Organization, said she was worried about attacks on cultural institutions and looting in the country.
"This constitutes irreversible damage to the history and identity of the Egyptian people," she said in a statement Monday.
Western governments have been reluctant to sever ties with the military-backed government because of strategic implications for the broader Middle East. Amnesty International said Monday the international response to the bloodshed has been "weak."