Mikhail Khodorkovsky was placed in an isolation cell for 12 days Wednesday, a week before hearings on his appeal against the denial of parole.
"The punishment of the ex-Yukos head for nonexistent violations in the run-up to the court hearings on his release on parole has already become an unhealthy tradition," the statement says.
The lawyers said they would seek a criminal investigation of the incident at Russia's top investigation body, the Investigation Committee of the Russian Prosecutor General's Office.
The Russian Federal Penitentiary Service was not immediately available for comment.
Since the oligarch's arrival at the prison in October 2005, he has been placed in solitary confinement several times, including on one occasion for his own safety after he had his nose slashed by a fellow inmate.
In June 2006 he was sent to an isolation cell for violating rules on the transfer of food or possessions between inmates.
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