Russian opposition leader Alexei Navalny on Thursday accused prison authorities of torture, claiming they have been depriving him of sleep, and said his health has been declining. The outspoken Kremlin critic, considered a flight risk by Russian authorities, claims guards wake him up eight times every night just for announing to a recording camera that he is still in his cell.

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“Essentially I am being tortured through sleep deprivation,” AFP quoted Navalny saying in a formal complaint released by his lawyers. 

Navalny is serving a two-and-a-half-year sentence on old embezzlement charges in a prison outside Moscow. 

The 44-year-old was arrested on his return to Russia in January from Germany, where he spent months recovering from a near-fatal nerve agent poisoning in August that he blames on the Kremlin.

His allies have raised concerns about his health after complaining of back and leg pain in recent days.

“I believe that my deteriorating health is the direct consequence of actions and inaction of FSIN employees,” Navalny said, referring to the prison service by its Russian acronym.

Navalny’s lawyers managed to visit him at his prison in the town of Pokrov around 100 kilometres (60 miles) east of Moscow on Thursday, after they were denied access to the penal colony the day before.

Lawyer Olga Mikhailova said he had been in pain for the past four weeks.

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“He is suffering from strong back pain and pain in his right leg,” she said in remarks on Dozhd television, adding that his condition was “extremely unfavourable”.

“Everyone is afraid for his life and health.”

Mikhailova said Navalny’s right leg was in “an awful condition” and that we was losing sensation in the limb.

She said that on Wednesday Navalny had undergone a “strange” MRI examination in a hospital outside the prison but doctors did not release any diagnosis.

She demanded he be freed and transferred to Moscow for treatment.

The prison service earlier on Thursday insisted that Navalny has been checked by medics and that “his state of health was assessed as stable and satisfactory”.

Navalny’s arrest and jailing sparked large-scale protests, an outcry from rights groups in Russia and abroad and condemnation from Western governments.

The United States and the European Union have slapped sanctions on Russia for jailing Navalny and for allegedly orchestrating last year’s poisoning, a charge Moscow has denied.

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The Kremlin said it was not following reports of Navalny’s health and had not requested information from prison authorities.

“The condition of convicts and people who are serving time in correctional institutions is being monitored by their administrations. That’s their job,” spokesman Dmitry Peskov told reporters.

Maria Pevchikh, who heads investigations at Navalny’s Anti-Corruption Foundation, said on Wednesday that supporters believed his “life is in danger”.

Navalny has been a thorn in the Kremlin’s side for a decade by probing corruption among officials and leading large protests throughout Russia.

His team this week launched a new campaign seeking his release and announced plans to stage what they said would be “modern Russia’s biggest protest” involving at least 500,000 people.