Russian opposition leader Alexei Navalny was rushed to hospital after a suspected poisoning on Thursday.
He was unconscious and in intensive care after his airplane made an emergency landing, according to his spokeswoman, Kira Yarmysh. He later fell into a coma, she said.
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."
She later posted that Alexei was in a coma and had been connected to a ventilator. She said the police had been called to the hospital at their request. Tass news agency reported that he was in a serious condition, citing the head doctor at Omsk Ambulance Hospital number 1.
Omsk supporters of Navalny shared unconfirmed footage of Navalny being carried off the plane in a stretcher.
The Omsk Oblast Health Ministry confirmed Navalny was in hospital.
"Today, at Omsk-Centralny Airport, an S7 aircraft with the famous blogger Alexei Navalny on board made an emergency landing. He was taken by ambulance to one of the city hospitals. At the moment Alexei Navalny is in intensive care, where he is receiving all necessary examinations," the ministry told Interfax.
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.