Doctors said the wounds of the inmates of Lgov prison in Kursk region were not life-threatening.
The regional prosecutor, Alexander Babichev, confirmed on national television that "260 inmates mutilated themselves" to demand improved conditions and the resignation of the prison authorities, the BBC reported Tuesday.
"Following checks, facts concerning illegal treatment of detainees have been established, and those that allowed these violations will be prosecuted," he said.
The self-mutilation began in 10 sections of the jail simultaneously Sunday night, investigators said.
Relatives of the prisoners spent Tuesday outside the jail demanding access to the inmates. Some reportedly threatened to start a hunger strike unless they got satisfactory answers.
Human rights activists said they will 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, saying inmates had "refused to accept legitimate demands of the prison authorities for the enforcement of detention rules," the Interfax news agency reported.