At least eight members of the Russian Interior Ministry's special police force, OMON, have been injured trying to restore order at a prison in Chelyabinsk in the Ural Mountains.
Details of what happened at the Kopeysk prison remain sketchy.
The press service for the local branch of the Interior Ministry said some 250 prisoners were able to reach the rooftops of some prison buildings late on November 24 and stage a protest.
The protesters were reportedly demanding the release of a group of prisoners from a special punishment cell.
Some of the prisoners held up banners that read "People, help."
Some 300 of the prisoners' relatives and former inmates gathered outside the prison walls and also staged a protest. Some of those outside the prison reportedly screamed obscenities and threw bottles at police who arrived to disperse the crowd.
Police detained 38 people outside the prison and said most of those taken into custody were drunk.
Situation Under Control
Human rights activist Oksana Trufanova told the Ekho Moskvy radio station that OMON troops attacked the group outside the prison. "They beat us with batons," Trufanova said. "Their eyes were shining. I have never been in such mayhem."
The local branch of the Interior Ministry denied OMON troops had beaten anyone. Some 250 police officers remain at the scene and the situation inside and outside the prison was reported to be under control.
Rights organizations said there had been complaints about the abuse of prisoners at the facility.
Lev Ponomarev, the head of the For Human Rights movement, described Chelyabinsk as a "hot spot" and said local prosecutors had not reacted to complaints from inmates at the Kopeysk prison.
Ponomaryov said that in 2008 four inmates at Kopeysk died after being beaten.
There are more than 700,000 inmates in Russian prisons.
Based on reporting by ITAR-TASS, Interfax, and Reuters