MOSCOW, RUSSIA -- Kremlin critic Alexei Navalny, who is serving a two-and-half-year jail term in a penal colony outside Moscow, said Monday he was locked up in a "real concentration camp".

His comments were the first confirmation of widespread reports that the Russian opposition politician would be spending his sentence at one of the most notorious facilities in Russia's extensive network of over 600 work colonies.

"I have to admit that the Russian prison system was able to surprise me. I had no idea that it was possible to arrange a real concentration camp 100 km from Moscow," Navalny posted on Instagram along with an old photo of himself with a close-cropped haircut.

He added that he was in Penal Colony No. 2 in the town of Pokrov in the Vladimir region northeast of Moscow with a "freshly shaven head".

Also on Monday, Russian news agencies cited Navalny's lawyer Olga Mikhailova as confirming that he was at the colony, saying that she had been able to visit him there.

In his post, Navalny wrote that "video cameras are everywhere, everyone is watched and at the slightest violation they make a report".

"I think someone upstairs read Orwell's 1984 and said: 'Yeah, cool. Let's do this. Education through dehumanisation'," he added.

Navalny said that he had not yet seen any hints of violence at the colony, but because of the "tense posture of the convicts", he can "easily believe" previous reports of brutality.

Earlier this month, activist Konstantin Kotov, who spent nearly two years at the colony for violating protest rules, described to AFP an environment in which inmates are not treated "like people".

In mid-January, Navalny was taken into police custody shortly after landing at a Moscow airport from Germany, where he had been treated for poisoning with the Soviet-era nerve toxin Novichok.

The anti-graft campaigner, who gained prominence for his investigations into the wealth of Russia's elites, insists the poisoning was carried out on the orders of President Vladimir Putin.

The Kremlin has repeatedly denied the claim, but is yet to launch a probe into the attack.