Russian opposition leader Alexei Navalny has said he being harassed by prison officials in detention and is being threatened with solitary confinement.
In a post on Instagram, Navalny said he has already received ten warnings for alleged bad behaviour. Also uploaded was an image of Navalny in prison uniform, with a shaven head.
"There are two main punishments in Russian prisons: a reprimand and placement in a punishment cell," Navalny said.
"You get two reprimands and you can go to the punishment cell, and it's not a pleasant thing, the conditions there are close to torture."
Navalny said he had received reprimands for leaving his bed ten minutes before the wake-up call and refusing to exercise.
"When they reprimand me sitting under a portrait of [Russian President Vladimir] Putin, I remember how they used to reprimand me at school during the staff meeting," he added.
Since his imprisonment, Navalny has regularly posted messages on social media, despite having no access to the internet.
The Kremlin critic was sent to a prison in the Vladimir region, east of Moscow, at the end of February for his two and a half year sentence.
Last week, he complained that prison authorities have ignored his requests to be examined by an independent doctor for "acute pain in his back". Navalny said the illness has spread to his right leg and prevents him from walking.
He has also accused prison officials of subjecting him to "sleep deprivation" torture by waking him up every hour each night.
Navalny's wife, Yulia Navalnaya, has called on Putin to "immediately" release the politician, although the Kremlin advised her to contact the prison services.
The opposition leader was poisoned by a nerve agent last August and accuses Putin of ordering his assassination by Russia's Federal Security Service. Moscow has denied any involvement.
Navalny was arrested in January as he landed back in Russia from Germany where he was recovering.
He was convicted for a 2014 fraud case, which NGOs and many European governments say is politically motivated.