Amnesty International says at least 208 people are believed to have died in Iran during clashes between security forces and demonstrators protesting a fuel hike.
The protests erupted in November and spread across the country. Iran's state-run media say 140 public facilities have been torched or damaged.
The London-based rights group released the number on Monday based on "credible sources" inside and outside Iran, including victims' relatives, journalists and activists.
Amnesty International said the death toll is a "further evidence that Iran's security forces went on a horrific killing spree."
It also condemned the idea that "families of victims have been threatened and warned not to speak to the media, or not to hold funeral ceremonies for their loved ones."
The former national security adviser for the US administration, John Bolton also criticized the Iranian government on Twitter on Monday.
Bolton who is known for his hardline stance on Iran said the "security forces opened fire on unarmed protesters in multiple locations."
He added, "There can be no negotiation, no trust, on nuclear issues with a government that slaughters its people."
The Iranian government admits there were casualties but disputes the death tolls of human rights groups as baseless.