The Russian military is accused of numerous abuses

The Memorial group has published details of the abandoned prison on its website, including photos and graffiti scrawled by inmates and their captors.

Former inmates told Memorial they had been tortured there.

The pro-Moscow Chechen authorities said their own investigators had "found no evidence of torture".

Separatist rebels have been fighting Russian forces and Moscow's Chechen allies in the devastated North Caucasus republic since 1994.

Beatings

The dungeon was in a former school for deaf children occupied by Russian interior ministry troops until late last month, Memorial reported.

Memorial activists visited the site in Grozny's Oktyabrsky district last week and photographed it.

Alavdi Sadykov, who spent three months in the prison in 2000, is missing his left ear. He told Memorial that his captors had cut it off.

Memorial says official investigations into his illegal detention and torture and disappearances of three other captives have dragged on for years and nobody has been punished yet.

"They would put you face down on a bench... then they placed a board on you and hit it with a hammer, beating your internal organs," Mr Sadykov said.

He said the idea was to avoid leaving external marks from the beating.

Official denial

The Chechen prosecutor-general has launched several criminal investigations in connection with the alleged disappearances and torture at the Oktyabrsky prison.

Human Rights Watch and other international human rights groups have accused Russian and pro-Moscow Chechen security forces of widespread abuses in Chechnya.

A human rights official working for the pro-Moscow administration in Chechnya, Nurdi Nukhazhiyev, rejected Memorial's allegations about the Oktyabrsky detention centre.

He told Russia's Itar-Tass news agency that the republic's investigators had "checked the temporary remand centre" and "found no evidence of torture".

"Had the allegation of Memorial corresponded to the facts, under no circumstances would this matter have been ignored by Chechen President Alu Alkhanov or Prime Minister Ramzan Kadyrov. But someone wants to create tension here, and I note this with regret," he said.