Russia opposition leader Navalny describes prison conditions Germany Kremlin Vladimir Putin Instagram Russia

Russian opposition leader Alexei Navalny described tight controls at his prison in a letter posted Monday, saying they include hourly checks during the night.

Navalny’s note posted on his Instagram page confirmed for the first time that he arrived at a prison colony in Pokrov in the Vladimir region, 85 kilometers (53 miles) east of Moscow which stands out among Russian penitentiary facilities for its particularly strict regime.

Navalny described the prison, IK-2, as a “friendly concentration camp.” He said that he hasn’t seen “even a hint at violence” there but faced overwhelming controls that he compared to George Orwell’s “Nineteen Eighty-Four.”

Navalny, 44, Russian President Vladimir Putin’s most vociferous foe, was arrested on Jan. 17 upon returning from Germany where he spent five months recovering from a nerve-agent poisoning that he blames on the Kremlin. Russian authorities have rejected the accusation.

Last month, Navalny was sentenced to 2 1/2 years in prison for violating the terms of his probation while convalescing in Germany. The sentence stems from a 2014 embezzlement conviction that Navalny has rejected as fabricated — and which the European Сourt of Human Rights has ruled to be unlawful.