Several hostage takers have been killed in an operation to free two prison guards who they were holding in the southern Russian city of Rostov, Russia media report.
The guards are said to be unhurt after special forces stormed the centre.
The six men, who were being held in a pre-trial detention centre, were said to be armed with knives.
They were facing terrorism charges, including links to the Islamic State (IS) group.
They had been demanding a vehicle, firearms and free passage out of the centre.
Police and national guard were called to the scene when reports of the hostage taking first emerged on Sunday morning.
In a video released by the prisoners, one of the hostages is heard to say the detainees have “serious intentions”.
The detainees said they were members of IS and that their actions were “not spontaneous”.
Reports say they managed to smuggle knives, IS flags and mobile phones into their cells.
They knocked out bars of a cell window and entered a guard room, where they were able to seize the hostages.
Several attacks claimed by IS have been carried out on Russian soil, most recently in March this year when gunmen opened fire in a concert hall in Moscow killing 145 people and injuring more than 500.
Russia tried to link the attack to Ukraine, but Kyiv swiftly denied any involvement.
