The opposition leader has been placed on a ventilator in intensive care after being taken ill during a flight to Moscow.
Doctors treating a leading Russian opposition activist, who is in a coma after a suspected poisoning, are “fighting to save his life”, one of them has said.
Alexei Navalny, 44, a long-time critic of Russian President Vladimir Putin, is unconscious in intensive care in a hospital in the city of Omsk, according to his spokeswoman Kira Yarmysh.
“He is in a coma in grave condition,” she said on Twitter, adding that he was placed on a ventilator.
A local health ministry official said Mr Navalny’s life was in danger, but that he was in a stable condition.
The pro-democracy campaigner was taken ill during a flight to Moscow from the city of Tomsk, his condition forcing the plane to make an emergency landing in Omsk, so he could receive medical attention.
A fellow passenger described hearing him “screaming in pain”.
Ms Yarmysh tweeted: “Alexei has a toxic poisoning.”
She added: “We assume that Alexei was poisoned with something mixed into the tea. It was the only thing that he drank in the morning.
“Doctors are saying the toxin was absorbed quicker with hot liquid,” she said, adding that Mr Navalny’s team called police to the hospital.
But Anatoliy Kalinichenko, deputy chief doctor at the Omsk hospital, would not confirm Ms Yarmysh’s assertion, saying poisoning was one of a number of causes medics were considering.
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Mr Kalinichenko said the activist was in a grave, but stable condition, but refused to give details because of confidentiality laws.
Mr Navalny is in a serious condition, the TASS news agency said, quoting the head doctor of the hospital.
A passenger on the flight, Pavel Lebedev, said: “At the start of the flight he went to the toilet and didn’t return.
“He was really sick and is still screaming in pain. They didn’t say what exactly happened to him. We landed in Omsk. Ambulance arrived.”
A staunch critic of the Kremlin, Mr Navalny was jailed for 30 days last year for violating strict protest laws and calling for unauthorised demonstration in Moscow which led to the arrests of more than 1,000 people.
Whilst being held, he contracted an illness diagnosed as “contact dermatitis”, but which he said could have been a poisoning.
He attempted to stand in the 2018 presidential race but was barred because of previous fraud convictions in a case he again said was politically motivated.
He has described the country’s main party, United Russia, which is led by President Vladimir Putin, as a “party of crooks and thieves”.
In June, the anti-corruption campaigner described a vote on constitutional reforms, which allow Mr Putin to serve another two terms in office, as a “coup” and a “violation of the constitution”.
Navalny’s Foundation for Fighting Corruption has been exposing graft among government officials, including some at the highest level.
Last week he was accused of stirring mass protests in Belarus by the country’s authoritarian leader, President Alexander Lukashenko.
In March 2018, Sergei Skripal, a former Russian spy, and his daughter, Yulia, were left critically ill after being targeted in Salisbury by Russian agents using novichok, a nerve agent, but recovered.
Mother-of-three Dawn Sturgess died from novichok poisoning after her partner Charlie Rowley found a contaminated bottle discarded by the perpetrators, believing it to be a perfume bottle.
Twelve years earlier, Russian defector Alexander Litvinenko died in London after being poisoned with radioactive polonium-210, believed to have been administered by agents working for the Kremlin.