Amnesty International says several people were killed in May when Iranian security forces used live ammunition to quell protests over the rising cost of living and has called on the international community to hold authorities to account.
The protests over rising food prices erupted in southwestern Khuzestan Province in early May before spreading to neighboring Chaharmahal and Bakhtiari provinces.
In a report published on August 3, Amnesty said video and audio evidence indicated that security forces "fired weapons loaded with live ammunition and birdshot on multiple occasions during the protests."
"The authorities' militarized response laid bare once again their utter disregard for the sanctity of human life and international legal standards on the use of force and firearms," said Amnesty's Diana Eltahawy.
Three people were killed in Chaharmahal and Bakhtiari provinces in connection with the protests between May 14 and 17, it said.
On May 15, a member of parliament also reported the death of another person in Khuzestan Province in connection with the demonstrations.
Protests also took place later in the month after the deadly collapse of a building in the Khuzestan city of Abadan, where authorities fired shots "at crowds of grieving protesters," the report said.
Amnesty said there were "cycles of protest bloodshed" in Iran with an "increasing militarization of the policing of protests."
During the investigation, Amnesty reviewed and analyzed extensive video footage, official statements, and state media reports and spoke to human rights defenders and journalists in contact with affected individuals and communities.
The group said Iranian authorities need to be held accountable for the "torrent of violence" unleashed against the protesters.
"Iran's security forces will continue to feel emboldened to kill and injure protesters if they are not held accountable," said Eltahawy, reaffirming Amnesty's demand for the UN to establish an independent mechanism to investigate such crimes in Iran.