Inmates at Iran's Evin Prison have been blindfolded and handcuffed before being dragged and beaten by baton-wielding guards, Amnesty International said in a report.
The international rights monitor said on Thursday that the events at the north Tehran jail on April 17, described as "Black Thursday" by local activists, represented an "unwarranted use of force".
"Security officials responded with an appalling level of brutality to the protest at Evin prison, dragging them along the floor and verbally insulting them," said Said Boumedouha, Amnesty's deputy director for the Middle East and North Africa.
Security officials responded with an appalling level of brutality to the protest at Evin prison, dragging them along the floor and verbally insulting them.
Amnesty said its report titled "Justice is an Alien Word: ill treatment of political prisoners in Evin Prison," documented dozens of inmates at the jail's Section 350 that were subjected to assault, beatings and other ill-treatment.
Some of those injured were denied access to adequate medical care, the organisation said in a statement.
"Prisoners were blindfolded and handcuffed before being shoved through a 'tunnel' formed of security officials carrying batons, who repeatedly struck them on their backs, heads and faces," it said.
At least 32 inmates were taken to a solitary confinement area shortly after the incident, Amnesty said, with some being forced to strip naked before being locked in cells.
Iran authorities said an official inquiry would be launched to probe claims that guards at Section 350, which houses political prisoners, had acted improperly.
"The justice and intelligence ministries will prepare a report about the incident at Evin," Vice President Mohammad Baqer Nobakht told ISNA news agency.
"We are waiting to receive that report and people will be informed about it."
At the time of the incident Iranian officials dismissed accounts published on opposition websites as inaccurate and said it was the prisoners who resisted an inspection and started a fight with the guards.