A plane carrying a prominent Russian politician who is in a coma after a suspected poisoning has left the Siberian city of Omsk for Germany, according to his spokeswoman. 

The plane took off just after 8am local time (02:00 GMT) on Saturday, after more than 24 hours of wrangling over Alexei Navalny's condition and treatment, with the opposition leader's allies accusing Russian authorities of trying to stop his evacuation.

Kira Yarmysh, Navalny's spokeswoman, said the politician was transferred to an ambulance early on Saturday and driven to the airport. 

"The plane carrying Alexei is flying to Berlin," she said on Twitter.

"Many thanks to everyone for their support. The fight for Alexei's life and health is just beginning and there is a long way to go, but at least the first step has been taken."

Navalny's wife, Yulia, was also on board the flight, Yarmysh said.

One of Russian President Vladimir Putin's fiercest critics, Navalny was admitted to an intensive care unit in Omsk on Thursday. His supporters believe that tea he drank was laced with poison - and that the Kremlin is behind both his illness and the delay in transferring him to a top German hospital.

Russian doctors say there is no evidence of poisoning, and the Kremlin denied the authorities tried to prevent the transfer from happening.

Even after German specialists arrived on a plane equipped with advanced medical equipment on Friday morning at his family's behest, Navalny's physicians in Omsk said he was too unstable to move.

Navalny's supporters denounced that as a ploy by authorities to stall until any poison in his system would no longer be traceable. The Omsk medical team relented only after a charity that had organised the medevac plane revealed that the German doctors examined the politician and said he was fit to be transported.

Deputy chief doctor of the Omsk hospital Anatoly Kalinichenko then told reporters that Navalny's condition had stabilised and that physicians "didn't mind" transferring the politician, given that his relatives were willing "to take on the risks". 

The Kremlin denied resistance to the transfer was political, with spokesman Dmitry Peskov saying that it was purely a medical decision. However, the reversal came as international pressure on Russia's leadership mounted.