Activists from the Memorial group said on June 7 they discovered the detention center in Grozny's southern Oktyabrsky district last week. They took photographs and video footage of the prison cell walls on which inmates had left various notes, including names and detention dates.
Yury Orlov, the group's head, said the detention center should have been closed several years ago, but apparently continued functioning until last month. Memorial has submitted the purported evidence to prosecutors.
Chechen prosecutor Valery Kuznetsov said a preliminary investigation had been launched into the claims. But Chechnya's ombudsman, Nurdi Nukhazhiev, denied the existence of any secret prison.
The International Helsinki Federation for Human Rights (IHF) last month alleged that Russian security services and their Chechen allies are operating clandestine prisons in the breakaway province.
The aftermath of a December 2002 Chechen resistance attack on the main government building in Grozny (epa)