Russian opposition leader Alexei Navalny has been moved to Germany from Siberia for medical treatment.

The prominent critic of President Vladimir Putin is in a coma after drinking what his supporters suspect was poisoned tea; they accuse the authorities of trying to conceal a crime.

Doctors treating Alexei Navalny in Omsk had insisted on August 21 that he was too ill to be moved.

However, they later said his condition was stable enough for the flight. His wife Yulia Navalnaya is traveling with him.

On August 22, Alexei Navalny’s medical evacuation flight – paid for by the German non-governmental organization Cinema for Peace – landed at Tegel airport in Berlin. He is being treated at the Charité hospital in the German capital.

The founder of the Cinema for Peace Foundation, activist and filmmaker Jaka Bizilj, told reporters outside the hospital that Alexei Navalny’s condition was “very worrying”.

“It’s not only about the question if he will survive this,” Jaka Bizilj said.

“It’s a question what kind of damage there is, if he will survive this and come back to normal fully.”

Alexei Navalny’s personal doctor, Anastasia Vasilieva, was not allowed to see him while he was in hospital in Russia. But she said she was hopeful he could recover now that he was in Germany.

She said: “I’m sure that they can treat him and do everything to eliminate this toxic agent from his body.”

Medical staff treating Alexei Navalny at the hospital in Omsk said before his flight that his life was not in immediate danger.

His spokeswoman, Kira Yarmysh, earlier tweeted: “Massive thanks to everyone for their support. The struggle for Alexei’s life and health is just beginning.”

Kira Yarmysh said it was a pity that doctors had taken so long to approve his flight as the plane and the right documents had been ready since August 21.

Alexei Navalny fell ill during a flight from Tomsk to Moscow on August 20, and his plane made an emergency landing in Omsk.

A photograph on social media appeared to show the Russian opposition leader drinking from a cup at a Tomsk airport cafe before the flight. His team suspects a poisonous substance was put in his tea.

Disturbing video appeared to show a stricken Alexei Navalny howling in agony on the flight.

Russia: Alexei Navalny In Coma After Allegedly Being Poisoned

The head doctor at the hospital where Navalny was being treated in Omsk, Alexander Murakhovsky, warned late on August 21 that doctors did not recommend flying “but his wife insists on her husband being transferred to a German clinic”.

“The patient’s condition is stable,” deputy chief doctor Anatoly Kalinichenko was quoted as saying by the Interfax news agency.

“As we’re in possession of a request from relatives to permit him to be transported somewhere, we have now taken the decision that we do not object to his transfer to another in-patient facility,” he added.

Doctors said earlier that no poison had been found in Alexei Navalny’s body, suggesting his condition might be the result of a “metabolic disorder” caused by low blood sugar.

Health officials then indicated that traces of an industrial chemical had been found on his skin and hair. The local interior ministry told the Rapsi legal news agency that the chemical was usually included in polymers to improve their elasticity, but its concentration was impossible to establish.

Alexei Navalny has consistently exposed official corruption in Russia. He has served multiple jail terms.

Foreign leaders including German Chancellor Angela Merkel and France’s President Emmanuel Macron have expressed concern for him.

In the US, Democratic presidential candidate Joe Biden has described the incident as “unacceptable” and vowed that, if elected, he would “stand up to autocrats like Putin”.