Some 260 inmates have slashed their wrists or necks at a prison in western Russia in a protest at their poor conditions and alleged mistreatment.
Their wounds, examined by doctors at the jail in Lgov in the Kursk region, were not said to be life-threatening.
Inquiries backed up inmates' claims of mistreatment and prosecutors said those responsible would be tried.
Earlier, they called the jail a "model institution" and stressed they had received no complaints about it before.
Regional prosecutor Alexander Babichev confirmed that "260 inmates mutilated themselves" to demand improved conditions and the resignation of the prison authorities.
"Following checks, facts concerning illegal treatment of detainees have been established and those that allowed these violations will be prosecuted," he said on national television.
Unconfirmed reports said more than 500 inmates took part in the self-mutilation, which happened on Sunday night.
A group of relatives spent the day outside the jail demanding access to the inmates.
One report said some threatened to go on hunger strike unless they got satisfactory answers.
Investigators said the protest was carefully planned - beginning in 10 sections of the jail simultaneously.
They stressed the inmates' wounds were superficial and had been treated by prison medics.
Human rights activists are planning to conduct their own investigation, and are sending a delegation from Moscow to Lgov.
The Russian prison service condemned the protest as an act of disobedience, in a statement quoted by Interfax news agency.
It said inmates had "refused to accept legitimate demands of the prison authorities for the enforcement of detention rules".