Russian opposition figure Alexei Navalny is unconscious in hospital suffering from 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.
The Kremlin said that it wished Mr Navalny a "speedy recovery".
Mr Navalny, 44, has been 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.
Russia's vociferous Putin critic
British Foreign Secretary Dominic Raab said he was "deeply concerned" by the reports Mr Navalny had been poisoned, and sent his thoughts to him and his family.
What has the spokeswoman said?
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."
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 on a ventilator and in a coma, and that the hospital was now full of police officers. She said they had later asked to search his belongings.
She also said that doctors were initially ready to share any information but then they later claimed the toxicology tests had been delayed and were "clearly playing for time, and not saying what they know".
She said in a tweet at 14:58 local time (08:58 GMT) that Mr Navalny's condition had not changed and he was still unconscious.
She was told the diagnosis would be "towards evening".
Both Mr Navalny's wife, Yulia Navalnaya and doctor, Anastasia Vasiliyeva, had arrived at the hospital but were being denied access to him, Ms Yarmysh said.
What are the other reports from the scene?
The Tass news agency quoted one source at the Omsk Emergency Hospital as saying: "Alexei Anatolyevich Navalny, born in 1976. Poisoning intensive care."
However, the deputy head physician of the hospital later told media that it was not certain Mr Navalny had been poisoned, although poisoning was "naturally" one of the diagnoses being considered.
Anatoly Kalinichenko said that doctors were "genuinely trying to save [Mr Navalny’s] life".
Kremlin spokesman Dmitry Peskov later said it wished the critic a speedy recovery – as it would all citizens in such circumstances – and that the authorities would consider approving treatment abroad if it were requested.
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