Orlov, a leader of Memorial which won a share of the Nobel Peace Prize in 2022, was sentenced last month to two-and-a-half years in prison for "discrediting the armed forces" after he took part in anti-war demonstrations and published an article in which he said Russia had descended into fascism.

He has since filed an appeal.

Memorial said Orlov's health was suffering due to a strict diet of "dry rations" added to boiling water. He is also forced at the request of the judge to make daily trips between the detention centre and the court to review the case materials for his appeal, travelling in an unheated and smoke-filled police van, the group said.

It also said he is not being granted regular and confidential access to his lawyers.

"Orlov's constant trips to court, as well as the conditions of transportation and detention during trips, together constitute cruel and degrading treatment and violate Orlov's rights," his defence team wrote in official complaints it filed to court and posted on Memorial's website.

"The current situation gives me serious fears for the life and health of my client," the complaint says.

Reuters has requested comment from the Russian prison service.

Memorial, founded in 1989 to document human rights abuses in the Soviet Union, has decried the case against Orlov as an attempt to "drown out the voice of the human rights movement in Russia."

The organisation said earlier this month that Orlov was asked in detention to sign a form indicating his willingness to fight in Ukraine, despite his age.

Orlov laughed off the request, Memorial said, writing "I don't agree."

Australian Associated Press