Amnesty International on Monday said that at least 304 people were killed and thousands more injured between November 15 and 18 as Iranian authorities "crushed protests using lethal force."
In a report published on Monday, the human rights group also said that authorities were carrying out a "vicious crackdown" following the demonstrations in order to scare people into not talking about what happened.
"Harrowing testimony from eyewitnesses suggests that, almost immediately after the Iranian authorities massacred hundreds of those participating in nationwide protests, they went on to orchestrate a wide-scale clampdown," Amnesty's Middle East and North Africa Research Director Philip Luther said.
Amnesty said that it verified footage, backed up by witness testimony, showing "Iranian security forces opening fire on unarmed protesters who did not pose any imminent risk."
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The United Nations said it had information suggesting that at least 12 children were among those killed.
Luther called on the international community to act and on the UN Human Rights Council to hold a special session on Iran "to mandate an inquiry into the unlawful killings of protesters, horrifying wave of arrests, enforced disappearances and torture of detainees."
Amnesty and the UN earlier this month had placed the death toll at 208, while the U.S. State Department's special representative for Iran said the number could be as high as 1,000.