Russian opposition leader Alexei Navalny is unconscious and in intensive care in hospital after apparently being poisoned, according to his spokeswoman.
Kira Yarmysh said on Thursday that Navalny was flying from Siberia to Moscow when his plane made an emergency landing after he fell ill.
"Alexei has toxic poisoning," she wrote on Twitter. "Alexei is now in intensive care."
"We think that Alexei was poisoned with something mixed in his tea. That was the only thing he drank in the morning," Yarmysh said.
"Doctors say the poison was quickly absorbed through the hot liquid," she added.
Navalny is in the intensive care unit for toxicology patients in Emergency Hospital No 1 in the Siberia city of Omsk, the TASS state news agency reported.
Russian opposition politician Alexei Navalny is in grave condition, TASS said, citing the head doctor of the hospital.
Past attacks
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.
Last year, Navalny was rushed to a hospital from prison where he was serving a sentence following an administrative arrest, with what his team said was suspected poisoning. Doctors then said he had a severe allergic attack and discharged him back to prison the following day.
He also endured chemical burns to his eye in 2017 when attackers threw green dye used as a disinfectant at his face outside his office.
Navalny, a charismatic lawyer and whistle-blower, has been 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."
