Federal investigators on Tuesday opened a criminal case in response to complaints by inmates at a Chelyabinsk region prison who staged a mass protest over the weekend.

At least 250 prisoners went on strike in maximum security prison number 6 on Friday in the town of Kopeisk. The strike was officially declared over on Monday.

The Investigative Committee said it had received complaints from inmates who said prison employees had been extorting money from them since 2008 by threatening violence or creating miserable living conditions. The extorted sums ranged from 5,000 ($161) to 90,000 rubles, the committee said.

During the strike, prisoners complained about harsh conditions and demanded that their cell mates be freed from disciplinary cells.

Investigators opened a criminal case with a charge of abuse of authority. They named no suspects in the case.

They also opened five criminal cases in connection with alleged attacks on riot police officers during a violent confrontation outside the prison over the weekend between law enforcement and prisoners' relatives.

Investigators said an inquiry into the reasons for the prisoners' protest is ongoing.

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