At least 76 protesters have been killed by Iranian security forces during 11 days of unrest sparked by the death of a woman in custody, activists say.
Iran Human Rights (IHR), a Norway-based organisation, accused authorities of using disproportionate force and live ammunition to suppress the dissent.
State media have put the number of dead at 41, including several security personnel, and blamed "rioters".
Hundreds of people have also been arrested, 20 of them journalists.
"The risk of torture and ill-treatment of protesters is serious and the use of live ammunition against protesters is an international crime, " said IHR's director Mahmood Amiry-Moghaddam. "The world must defend the Iranian people's demands for their fundamental rights."
The UN human rights office also said it was very concerned by the authorities' violent response and urged them to respect the right to protest peacefully.