Alexei Navalny broke his silence from Russian prison, describing 24/7 surveillance and being woken up every hour

Alexei Navalny, Putin's chief domestic critic, is serving 2 1/2 years in a Russian prison.

He was sentenced for missing parole hearings while he recovered from Novichok poisoning.

He spoke out from prison in a Monday Instagram post, comparing the prison to a "concentration camp."

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Alexei Navalny has for the first time revealed details about his life in Russian prison, reporting 24/7 surveillance and likening it to a concentration camp.

Navalny, the chief domestic critic of President Vladimir Putin, began a 30-month stint in prison after being sentenced on February 2 for missing parole meetings while abroad.

The activist had been receiving specialist medical care in Berlin after being poisoned with the Novichok nerve agent in August, for which he has blamed Putin. He was immediately arrested after returning to Russia in January.

Navalny's whereabouts after his sentencing were unknown until Russian authorities announced on February 28 that they had sent Navalny to Penal Colony No. 2 in the Vladimir region of Russia, east of Moscow.

Navalny had not been heard from until Monday, when he broke his silence on Instagram and described his situation.

"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 kilometers away from Moscow," he said.

A post shared by Алексей Навальный (@navalny)

"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.'"

Navalny said that at night he was woken up every hour by a prison guard holding a video camera, checking that he had not escaped.

"Three things never cease to amaze me," he added. "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."

Reports have detailed how prisoners in Russia are often tortured and subject to inhumane treatment.

Russian President Vladimir Putin on June 30. Alexei Druzhinin/TASS via Getty

Konstantin Kotov, a former inmate of Penal Colony No. 2 who was released in December, told CNN: "From the first minutes you are here you are experiencing mental and moral pressure."

He continued: "You are forced to do things that you would never do in normal life. You are forbidden to talk with other convicts. They force you to learn the list of names of the employees. You are on your feet all day, from 6 a.m. to 10 p.m. You are not allowed to sit down. They do not allow you to read, they do not allow you to write a letter. It can last two weeks, it can last three weeks."

Kotov said orderlies at Penal Colony No. 2 often punished inmates by hitting a chair leg against the underside of their feet.

Navalny said in his Instagram post that he had yet to see violence between guards and prisoners but believed reports of mistreatment given what he described as the "tense posture of the convicts."

Kotov added that he slept in a room with 50 to 60 metal bunks and spent almost every day watching Russian state TV. "This is torture by TV," he told CNN.

Large protests broke out across Russia in the wake of Navalny's detention, but they died down in recent weeks after a crackdown by law enforcement.

Thousands of people have been arrested, including Navalny's wife, Yulia Navalnaya.