Russian opposition figure Alexei Navalny is unconscious in hospital with suspected poisoning, his spokeswoman has said.
The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in Omsk, Kira Yarmysh said, adding that they suspected something had been mixed into his tea.
A hospital source confirmed treatment for poisoning to the Tass news agency.
Mr Navalny, 44, is a staunch critic of President Vladimir Putin.
In June he described a vote on constitutional reforms as a "coup" and a "violation of the constitution". The reforms allow Mr Putin to serve another two terms in office.
What did the spokeswoman say?
Kira Yarmysh, the press secretary for the Anti-Corruption Foundation, which Mr Navalny founded in 2011, tweeted: "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."
Ms Yarmysh later tweeted that Mr Navalny was in intensive care, on a ventilator and in a coma, and that the police had been called.
She said doctors were initially ready to share any information but were now saying the toxicology tests were delayed and were "clearly playing for time, and not saying what they know".
Tass reported that the Omsk Emergency Hospital had confirmed he was in a serious condition.
"Alexei Anatolyevich Navalny, born in 1976. Poisoning intensive care," the hospital source quoted by Tass said.
Images on social media showed Mr Navalny being taken on a stretcher to an ambulance on the airport runway.
Who is Alexei Navalny?
He made a name for himself by exposing official corruption, labelling Mr Putin's United Russia as "the party of crooks and thieves", and has served several jail terms.
In 2011 he was arrested and imprisoned for 15 days following protests over vote-rigging by Mr Putin's United Russia party in parliamentary elections.
Mr Navalny was briefly jailed in July 2013 on embezzlement charges but denounced the sentence as political.
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.
Mr Navalny was also given a 30-day jail term in July 2019 after calling for unauthorised protests.
He was taken ill during that jail sentence. Doctors diagnosed him with "contact dermatitis" but he said he had never had any acute allergic reactions and his own doctor suggested he might have been exposed to "some toxic agent". Mr Navalny also said he thought he may have been poisoned.
Mr Navalny also suffered a serious chemical burn to his right eye in 2017 after he was assaulted with antiseptic dye.
BBC
Russia's unmanned Luna-25 spacecraft has crashed into the Moon after spinning out of control, officials say.
Donald Trump has confirmed that he will not take part in Republican presidential debates with his rivals in the race for the White House.