Inmates at Iran's Evin Prison were blindfolded and handcuffed before being dragged and beaten by baton-wielding guards, Amnesty International said in a report released Thursday.
The international rights monitor said the events of April 17 at the north Tehran jail, 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.
Amnesty said its report, "Justice is an Alien Word: ill treatment of political prisoners in Evin Prison," documented that dozens of inmates at the jail's Section 350 were subjected to assault, beatings and other ill-treatment.
Some of those injured were denied access to adequate medical care, Amnesty said in a statement launching the report.
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"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 inmtes were taken to a solitary confinement area shortly after the incident, Amnesty said on Thursday, with some being forced to strip naked before being locked in cells.
Iran on Wednesday said an official inquiry would 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.