Russian opposition leader Alexei Navalny is unconscious and in intensive care in hospital after apparently being poisoned, his spokeswoman said Thursday.
Kira Yarmysh said Navalny was flying from Siberia to Moscow and his plane made an emergency landing after he fell ill.
Yarmysh told Echo of Moscow popular radio station: "I'm sure it was intentional poisoning."
"Alexei has toxic poisoning," she wrote on Twitter. "Alexei is now in intensive care."
Navalny is in the intensive care unit for toxicology patients in Omsk Emergency Hospital No. 1, the TASS state news agency confirmed.
"He is in a serious condition," the hospital's chief doctor Alexander Murakhovsky told TASS.
"Alexei is still unconscious. They have connected him up to a ventilator. They have called police to the hospital on our request," Yarmysh tweeted.
She wrote: "We think that Alexei was poisoned with something mixed in his tea. That was the only thing he drank in the morning."
"Doctors say the poison was quickly absorbed through the hot liquid," she added.
The 44-year-old, known for his anti-corruption campaigns against top officials and outspoken criticism of President Vladimir Putin, has suffered physical attacks in the past.
He endured chemical burns to his eye in 2017 when attackers threw green dye used as a disinfectant at his face outside his office.
In August last year Navalny suffered rashes and his face became swollen while he was in a police detention centre serving a short term for calling for illegal protests.
- Tactical voting campaign -
He was taken to hospital where doctors said he had suffered an allergic reaction but Navalny asked for an investigation into poisoning.
"He was poisoned in the police detention centre. I'm sure that now the same thing happened. It's different symptoms, evidently a different drug," Yarmysh told Echo of Moscow.
She told the radio station that she met Navalny to go to the airport in the Siberian city of Tomsk on Thursday morning when he seemed "absolutely fine."
"He only drank black tea in the airport," she said.
"Straight after takeoff he quite quickly lost consciousness."
Life News tabloid website posted a video shot on the plane with loud groans audible.
Telegram channel 112 posted a video shot from the plane of him being wheeled on a stretcher into an ambulance.
Navalny, a charismatic lawyer and whistleblower, is travelling the country to promote a tactical voting strategy to oppose pro-Putin candidates in more than 30 regional elections in September.
He has been the target of multiple criminal probes while his Anti-Corruption Foundation is regularly raided by police and investigators.
He has served numerous terms in police cells for organising illegal protests.
Yarmysh said they had been on a work trip to Tomsk.
Former US Ambassador to Moscow Michael McFaul tweeted: "Praying @navalny will be ok."