A video posted by one of the news channels, Mash, on Friday showed at least two attackers.
One of them shouted they were "mujahideen" of the militant group Islamic State and had seized control of the prison in the Volgograd region.
At least four uniformed prison officials could be seen lying or sitting in pools of blood.
Three were lying motionless, one with his throat cut.
Another was sitting upright in a doorway, where a man with a knife was holding him by the neck.
Russia state media said at least one member of the prison staff had been killed, but the video suggested the death toll was almost certainly higher.
The prison service said in a statement: "During a session of a disciplinary commission, convicts took staff of the penal institution as hostages. Measures are currently being taken to free the hostages. There are casualties."
The prison is designated as a "harsh regime" penal colony with capacity to hold up to 1241 male prisoners.
News sites with security connections published the names of up to four alleged attackers, identifying them as citizens of Uzbekistan and Tajikistan.
There was no official confirmation.
Russia, whose defence and security agencies are heavily focused on its war in Ukraine, has seen a recent upsurge in Islamist militant attacks.
In June, a bloody Islamic State-linked prison uprising took place in the southern region of Rostov, where special forces shot dead six inmates who had taken hostages.
Later that month, at least 20 people were killed in shooting attacks in two cities in Dagestan, a mainly Muslim region of southern Russia.
In March, Islamic State claimed responsibility for an attack in which gunmen raided the Crocus City concert hall near Moscow, sprayed the audience with automatic weapons fire and set fire to the building, killing more than 140 people.
