Amnesty International has accused the Iranian security forces of killing 23 children, mostly with live ammunition, during a crackdown on anti-government protests late last year.
The authorities launched a crackdown after protests erupted across Iran from mid-November following a surprise petrol price rise.
Amnesty has already said the crackdown left 304 people dead, a figure vehemently disputed by Tehran.
The human rights group said in a new report that it had evidence that at least 23 children were killed, with 22 of them killed by the security forces "unlawfully firing live ammunition at unarmed protesters and bystanders."
The children killed included 22 boys, aged between 12 and 17, and a girl reportedly aged between eight and 12.
"There must be independent and impartial investigations into these killings, and those suspected of ordering and carrying them out must be prosecuted in fair trials," said Philip Luther, Amnesty's research and advocacy director for the Middle East and North Africa.
Twelve of the 23 deaths - recorded in 13 cities in six provinces across the country - took place on 16 November, a further eight on 17 November, and three on 18 November, according to Amnesty.
"The fact that the vast majority of the children's deaths took place over just two days is further evidence that Iranian security forces went on a killing spree to quash dissent at any cost," Mr Luther said.
Amnesty International said it had written to Iran's Interior Minister Abdolreza Rahmani Fazli with the names of the 23 children recorded as killed but had received no response.
It said relatives of some of the children killed described being subjected to harassment and intimidation, including surveillance and interrogations by intelligence and security officials.
It said this corresponded with a broad pattern of families of those killed in protests being intimidated by the state to prevent them talking openly about the deaths.
"Families of children killed during the protests are facing a ruthless campaign of harassment to intimidate them from speaking out," said Mr Luther, denouncing a "state cover-up".
Its report was based on evidence from videos and photographs, as well as death and burial certificates, accounts from eyewitnesses and victims' relatives as well as information gathered from human rights activists and journalists.