Russian opposition politician Alexei Navalny is fighting for his life in a Siberian hospital after suffering severe symptoms of what his spokeswoman says is believed to be a deliberate poisoning.

Navalny, 44, is unconscious, in intensive care and on an artificial lung ventilator, spokeswoman Kira Yarmysh said on social media on Thursday.

“We assume that Alexei was poisoned with something mixed into his tea,” Ms Yarmysh said.

“It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid. Alexei is now unconscious.”

Russian President Vladimir Putin. Photo: AAP

Navalny, a fierce critic of President Vladimir Putin, started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning, she added.

He had drunk tea at a cafe at Tomsk airport before boarding his flight. The Interfax news agency quoted the cafe’s owners as saying they were checking CCTV cameras in a bid to establish what had happened.

Mr Navalny’s plane made an emergency landing in Omsk so that he could be rushed to hospital, Yarmysh said.

Ms Yarmysh drew a parallel with an incident last year in which Navalny suffered an acute allergic reaction that one doctor said could have resulted from poisoning with an unknown chemical.

“Obviously the same has been done to him now,” she said.

Ms Yarmysh did not say who she believed may have poisoned Navalny, but said police had been called to the hospital.

There's still a lot we don't know. But: last year, Navalny was poisoned in jail. Three years ago, government-backed thugs sprayed him with "brilliant green," which resulted in a chemical burn to one of his eyes. https://t.co/dUsxrSRLhG

— Julia Ioffe (@juliaioffe) August 20, 2020

Navalny, a lawyer and anti-corruption activist, has served several stints in jail in recent years for organising anti-Kremlin protests.

He has helped release high-impact investigations into what he has said are outrageous examples of official corruption.

Russia holds regional elections next month and Navalny and his allies have been preparing for them, trying to increase support for candidates which they back.

The European Court of Human Rights has ruled that Russia’s arrests and detention of Navalny in 2012 and 2014 were politically-motivated and violated his human rights, a ruling Moscow called questionable.

-with AAP