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," Navalny posted on Instagram along with an old photo of himself with a close-cropped haircut.

"I had no idea that it was possible to arrange a real concentration camp 100 km from Moscow."

'I think someone upstairs read Orwell's 1984,' wrote Navalny
'I think someone upstairs read Orwell's 1984,' wrote Navalny AFP / Dimitar DILKOFF

Navalny 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 Monday, Navalny's lawyer Olga Mikhailova confirmed that he was at the colony, saying that she had been able to visit him there, Russian news agencies reported.

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 a prison guard woke him every hour at night
Navalny said a prison guard woke him every hour at night AFP / Dimitar DILKOFF

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".

Navalny is being held in Penal Colony No. 2
Navalny is being held in Penal Colony No. 2 AFPTV / Nikolay KORZHOV

In February, Europe's rights court told Moscow to release the opposition politician out of concern for his life, a call Russia swiftly rejected.

In his Instagram post, Navalny said that at night he was woken up "every hour" by a man who snaps a photo of him and announces that the convict who is "prone to escape" is still in his cell.

In mid-January, the Kremlin critic was taken into police custody shortly after landing at a Moscow airport from Germany, where he had been treated for a near-fatal 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 has yet to launch a probe into the attack.

Navalny's arrest set off a wave of protests across Russia and a brutal police crackdown. The United States and the European Union have called for his release.

In a coordinated action earlier this month, Washington and Brussels imposed sanctions on senior Russian officials, as US intelligence concluded that Moscow orchestrated the poisoning attack on Navalny.

A number of Navalny's close allies remain under house arrest, charged with violating Moscow's ban on mass events due to the coronavirus pandemic by calling for protests in his support.

Volkov, who is based in Lithuania, told AFP last week that the team would soon announce new protests for the spring and summer ahead of key parliamentary elections in the fall.