Several hostage-takers were killed during an operation to free two prison guards they were holding in the southern Russian city of Rostov, Russian media report.
The guards were not believed to be injured after the special forces stormed the center.
The six men, detained in a pre-trial detention center, are said to be armed with knives.
They faced terrorism charges, particularly in connection with the Islamic State (IS) group.
They demanded a vehicle, firearms and free passage out of the center.
Police and the National Guard were called to the scene when reports of a hostage situation emerged Sunday morning.
In a video released by the prisoners, one of the hostages can be heard saying that the detainees have “serious intentions”.
The detainees said they were members of ISIS and that their actions were “not spontaneous.”
Reports indicate that they managed to smuggle knives, IS flags and cell phones into their cells.
They knocked down the bars of a cell window and entered a guard room, where they were able to seize the hostages.
Several attacks claimed by ISIS have been carried out on Russian soil, the latest in March when gunmen opened fire in a Moscow concert hall, killing 145 people and injuring more than 500.
Russia tried to link the attack to Ukraine, but kyiv quickly denied any involvement.
