Alexei Navalny, one of the most prominent domestic opponents of Russian president Vladimir Putin, is in a coma in hospital. His supporters suspect he has been poisoned.

Navalny fell ill on a flight from Tomsk to Moscow, forcing the plane to make an emergency landing in Omsk.

Upon his arrival at hospital, Navalny was immediately rushed to intensive care and put on a ventilator after falling unconscious.

The 44-year-old's spokeswoman, Kira Yarmysh, who works for Navalny's Anti-Corruption Foundation, revealed his plight in a series of tweets on Thursday morning.

"This morning Navalny was returning to Moscow from Tomsk.

"During the flight, he felt ill. The plane made an urgent landing in Omsk. Alexei has toxic poisoning. Right now we are going to hospital."

She added: "We suspect that Alexei was poisoned by something mixed into [his] tea. It was the only thing he drank since morning.

"Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious."

Tatyana Shakirova, a spokeswoman for the regional health ministry in Omsk, said: "We confirm that Alexei Navalny has been hospitalised in Omsk and that doctors assess his condition as serious...Doctors are doing everything possible to stabilise his condition."

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

Footage on social media footage shows Navalny being rushed into an ambulance on a stretcher, while a passenger who was on the same plane revealed details from the flight. 

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."

Navalny, a lawyer and anti-corruption activist, has served many stints in jail in recent years for organising anti-Kremlin protests and has been physically attacked in the street by pro-government activists. He was previously hospitalized in July 2019 while in police custody, officially as a result of an allergic reaction. He subsequently suggested the incident may have been a poisoning.

Navalny has helped 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 Russian state news agency Tass reported a law enforcement source saying deliberate poisoning was not being considered now: 

 "While this version is not being considered, it is possible that he drank something or took it himself yesterday," Tass quoted  the source as saying.

Additional reporting from Reuters