In this Feb 29, 2020, file photo, Russian opposition activist Alexei Navalny takes part in a march in memory of opposition leader Boris Nemtsov in Moscow, Russia. Navalny has been poisoned and hospitalized on Aug 20, 2020 his spokeswoman Kira Yarmysh said on Twitter, Navalny felt unwell on a flight back to Moscow from Tomsk, a city in Siberia. (PAVEL GOLOVKIN / AP)

MOSCOW - Russian opposition politician Alexei Navalny was in a coma in a Siberian hospital on Thursday after drinking a cup of tea that his spokeswoman said she believed was laced with poison.

Alexei Navalny was in intensive care and on an artificial lung ventilator in an Omsk hospital, his spokeswoman Kira Yarmysh said

Navalny started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning. He was carried off the plane on a stretcher after it made an emergency landing at Omsk.

He was in intensive care and on an artificial lung ventilator in an Omsk hospital, his spokeswoman Kira Yarmysh said.

“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,” 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.

Anatoliy Kalinichenko, deputy chief doctor of the Omsk hospital of intensive care where Alexei Navalny is hospitalized speaks to the media in Omsk, Russia, Aug 20, 2020. (EVGENIY SOFIYCHUK / AP)

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

A regional health ministry spokeswoman, Tatyana Shakirova, confirmed that 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.”

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

SCREAMING IN PAIN

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’s plane later made an emergency landing in Omsk. Footage posted on social media showed a motionless Navalny being stretchered into an ambulance by medics.

S7, the airline he was travelling with, said 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.

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

She 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,” Yarmysh said.

Lithuanian Foreign Minister Linas Linkevicius said he was very worried about the possibility Navalny may have been poisoned. “If confirmed, those responsible must face consequences,” he said on Twitter.