At least 250 prisoners were on strike in the Chelyabinsk region for a second day Sunday, demanding a more relaxed prison regime.
The inmates refused to follow the orders of prison officials Saturday at maximum security prison number 6 in the town of Kopeisk, the Investigative Committee said in a statement.
Prisoners voiced "baseless" demands that prison officials relax the conditions at the prison, federal investigators said, adding that an inquiry into the protest is underway.
Regional prison officials said in a statement that the inmates were also demanding that their cell mates be freed from disciplinary cells.
Local human rights activist Oxana Trufanova estimated the number of protesting inmates at 300 and said they were on a hunger strike, Gazeta.ru reported. She said the prisoners were also protesting the beating of other inmates by prison officials.
According to Trufanova, who told the news site that she had met with officials of the prison on Saturday, the inmates started the strike Friday and intentionally stood for more than 24 hours outside in the cold from Friday to Saturday.
On Saturday, a group of inmates climbed a prison watch tower, hanging a banner that read,"People, help!" Trufanova said.
The case has been taken under the control of the Kremlin human rights council, said Vladimir Osechkin, head of the Gulagu.net prisoner defense group, to Gazeta.ru on Saturday.
Relatives of the inmates and human rights activists who gathered outside the prison for several hours Saturday told RIA-Novosi that riot police attacked people outside the prison with rubber batons for no apparent reason.
Police detained 38 people, many of them young and drunk, outside the prison for various administrative offenses, including trying to violate public order and swearing loudly, regional police said in a statement Sunday. The statement also said eight riot police officers had been injured during the detentions.
Veteran human rights activist Lev Ponomaryov called on the leadership of the Federal Prison Service to send an independent commission to the prison to investigate possible violations of the inmates' rights, Interfax reported.
In May 2008, 18 prison officials severely beat 12 new inmates, four of them to death, in penal colony number 1 in Kopeisk.
Officials then lied to investigators that the inmates had attacked them. In May 2011, eight prison officials were sentenced to various prison terms while the other 10 received suspended sentences.