Russian opposition politician Alexei Navalny is in a coma after drinking a cup of tea that his spokeswoman says is believed to have been laced with poison.

A fierce critic of President Vladimir Putin, Mr Navalny started feeling ill mid-flight when returning to Moscow from Siberia by plane on Wednesday morning.

One passenger reported him falling ill and screaming in pain during the flight and going to the toilet and not returning to his seat.

The plane was forced to make an emergency landing at Omsk and Mr Navalny was carried off on a stretcher.

His spokeswoman Kira Yarmysh said Mr Navalny was in intensive care and on an artificial lung ventilator in an Omsk hospital.

“We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Alexi is now unconscious,” Ms Yarmysh said.

Doctors gave contradictory information about his condition, saying it had stabilised but also that there was still a threat to his life and they were working to save him.

An ambulance aircraft with a team specialised in treating coma patients is due to leave Germany to pick up stricken the Russian dissident, the Berlin-based Cinema for Peace Foundation says.

There is a long history of Kremlin foes being poisoned or falling ill after suspected poisonings.

They include Alexander Litvinenko, who died in London in 2006 after drinking tea laced with polonium-210, and Sergei Skripal, a former double agent who was poisoned with a nerve agent in 2018 in Salisbury, England.

The Kremlin has repeatedly denied involvement in those and other incidents, calling them anti-Russian provocations.

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

A regional health ministry spokeswoman, Tatyana Shakirova, confirmed that Mr Navalny has been hospitalised in Omsk and said doctors assessed his condition as serious.

“Doctors are doing everything possible to stabilise his condition,” she said.

“The poisoning version is one of several versions being considered. It’s not possible now to say what the reason was.”

A woman expresses support for the opposition leader after he was rushed to intensive care in Siberia. Photo: Getty

Mr Navalny, a 44-year-old lawyer and anti-corruption activist, has served many stints in jail for organising anti-Kremlin protests and has been physically attacked in the street by pro-government activists.

He has helped investigations into what he has said are outrageous examples of official corruption, with his videos on the subject garnering millions of views.

Russia holds regional elections next month and Mr Navalny and his allies have been working to increase support for candidates whom they back.

Passenger Pavel Lebedev posted an account of what he saw on social media.

“At the start of the flight he went to the toilet and didn’t come back. He started feeling really sick. They struggled to bring him round and he was screaming in pain.”

S7, the airline he was travelling with, said Mr Navalny had started feeling very ill soon after take-off and the captain had decided to make an emergency landing at a nearby airport.

He had not eaten or drunk anything on board, it said.

Mr Navalny had a cup of tea at a Tomsk airport cafe before boarding his flight, Ms Yarmysh said.

She drew a parallel with an incident last year in which Mr 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,” Ms Yarmysh said.