Kobzev told news agency Interfax he had wanted to meet Navalny, who was being held in custody in Moscow, but was told that the opposition politician had been moved.

"I was not told where he was brought, likely to a prison camp, but possibly also elsewhere," Kobzev said.

There was initially no confirmation by official authorities.

"The fact that we now do not know where he is and what is happening to him confirms the scale of the threat," tweeted Leonik Volkov, an ally of Navalny who lives abroad.

Navalny was set to be sent to a penal camp after a court in Moscow confirmed his prison sentence on Saturday.

His lawyers believe he could be released in 2023 with time spent in detention and under house arrest taken into account.

Navalny was sentenced for violating parole in an earlier case as he was recovering from a nerve agent attack in Germany.

Upon his return to Russia, Navalny was immediately arrested and remanded in custody before being sentenced in an emergency trial.

The sentence has been criticised as politically motivated.

Australian Associated Press