Alexei Navalny, Russia’s president Vladimir Putin’s most outspoken critic, is said to be in a “very worrying” condition in hospital in Berlin, where he is being treated for suspected poisoning.

Navalny was flown from hospital in Omsk early on Saturday morning after his wife and closest supporters were able to gain permission from the Russian authorities, after 12 hours of delay.

The founder of the charity that organised the flight to Germany, Jake Bizilj, told reporters outside the Charite hospital: “We got a very clear message from the doctors that if there had not been an emergency landing in Omsk, he would have died.”

Navally was taken seriously ill on a flight from Tomsk, Siberia, en route to Moscow. His spokeswoman, Kira Yarmysh, said that he had drunk a cup of black tea at the Vienna coffee house at the airport, the only food or drink he had taken that day.

Local disc jockey Pavel Lebedev took a photo of Lebedev, who is well-known in Russia, about to drink from a paper cup.

Poisoning is a favoured assassination tool of the Russian authorities. It has also not escaped the attention of Kremlin watchers that Thursday marked the 80th anniversary of the assassination of Leon Trotsky on the direct orders of Jozef Stalin. Navalny remains in a coma.

With reporting by Leopold Kraus Photograph: DJpavlin/Instagram