Russian opposition leader Alexei Navalny compared his detention conditions to those of a "concentration camp" in an Instagram post on Monday.
"I must admit that the Russian prison system managed to surprise me. I did not think it was possible to build a concentration camp just 100 kilometres away from Moscow," the post read.
"Three things never cease to amaze me. The stars in the sky above us, the categorical imperative within us and the amazing feeling when you run your palm over your freshly shaven head," Navalny went on.
Navalny was arrested on January 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 two and a half years in prison for violating the terms of his probation while convalescing in Germany. He has dismissed the charge as a Kremlin vengeance.
His arrest and imprisonment triggered a wave of protests across Russia, to which the authorities responded with a massive crackdown.
While officials haven't said what prison he was sent to, Russian media reported it was in the city of Pokrov, 85 kilometres east of Moscow.
The prison has a reputation for its harsh detention conditions. Former inmates said they faced hourly check-ups, including all night, and would be banned from speaking to others.
On his Instagram post on Monday, Navalny said:
"Video cameras are everywhere, everyone is watched and at the slightest infringement, they make a report. I think someone up there read Orwell's 1984 and said: Yeah, cool. Let's do this."