Alexei Navalny, a key Russian opposition leader, was rushed to hospital after a suspected poisoning on Thursday.
He was unconscious in intensive care after his airplane made an emergency landing, according to his spokeswoman, Kira Yarmysh. He later fell into a coma.
He started feeling unwell on a flight from the Siberian city of Tomsk to Moscow, she said. The plane landed in Omsk and he was taken to hospital.
"We assume that Alexei was poisoned with something mixed with his tea," she said. "That's the only thing he drank this morning."
Alexei was later confirmed to be comatose and was connected to a ventilator. Doctors said he was a serious but stable condition and were investigating whether he had been poisoned.
Medical authorities told Interfax news agency that tests had shown he was poisoned with an unidentified psychodysleptic (hallucinogen), but this was unconfirmed by other sources.
Omsk supporters of Navalny shared unconfirmed footage of Navalny being carried off the plane on a stretcher.
'We have to save him'
Navalny's normal doctor told DW that the hospital was refusing to talk to her.
"The doctors have refused to speak to me," Anastasia Vasilieva told DW's Russian department. "I'm going to fly to Omsk now and insist that they allow me to examine him. We will decide whether he needs to be transferred to another hospital. We have to save him," she said of Navalny.
Vasilieva added that due to Navalny's serious condition and reports that he is in a coma, transferring him to a hospital abroad is currently not an option.
Navalny spends his time exposing corruption among the Russian elite and has suffered a constant campaign of intimidation and threats.
He suffered a similar incident last year where he was also suspected to have been poisoned while in police custody.
Kremlin wishes him well
A Kremlin spokesman said they wish him a speedy recovery as they would to "any other citizen who finds themselves in similar circumstances."
The European Union's foreign affairs chief Josep Borrell expressed over the suspected poisoning.
"Worried to hear about Alexei Navalny's ànavalny suspected poisoning," he said on Twitter. "If confirmed, those responsible must be held to account. Wishing him a swift and full recovery."
The management of a coffee shop on the second floor of Tomsk airport told Interfax it had initiated an internal investigation.
"Now we are investigating all the circumstances, we will study the surveillance footage," the director and owner of the coffee house said.
S7 airline told Interfax that Navalny did not eat or drink anything on board the flight.
Pavel Lebedev, a fellow passenger on the flight, described the scene in a post 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," he said.
aw/ng (Reuters, AFP, Interfax, AP)