One of Russia's most prominent human rights activists is accusing the Russian prison system of using hardened criminals to torture other inmates to extract confessions.

Lev Ponomaryov says his organization, For Human Rights, has received many letters from prisoners reporting abuse.

He told journalists Tuesday that some of the toughest criminals are ordered to torture, rape and sometimes kill other inmates.

A spokesman for the prison guard service denied the allegations. Spokesman Yevgeny Saurin said critics have failed to produce evidence to back up their claims.

Also Tuesday, President Dmitry Medvedev appointed a new director of the prison guard service. Human rights activists said they were eager to meet with him.