Russian opposition politician Alexei Navalny was in a coma in a Siberian hospital on Thursday after drinking a cup of tea that his spokeswoman said she believed was laced with poison.
Navalny, 44, was in intensive care and on an artificial lung ventilator, spokeswoman Kira Yarmysh said on social media.
"We assume that Alexei was poisoned with something mixed into his tea. It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid. Alexei is now unconscious," Yarmysh said.
She did not say who she believed may have poisoned Navalny, but said police had been called to the hospital. Medical tests were expected later on Thursday.
Navalny, a fierce critic of President Vladimir Putin, started feeling ill when returning to Moscow from Tomsk in Siberia by plane on Wednesday morning, Yarmysh said.
Navalny, a lawyer and anti-corruption activist, has served many stints in jail in recent years for organising anti-Kremlin protests and been physically attacked in the street by pro-government activists.
He has helped investigations into what he has said are outrageous examples of official corruption.
Russia holds regional elections next month and Navalny and his allies have been preparing for them, trying to increase support for candidates which they back.
Pavel Lebedev, a fellow passenger, posted an account of what he saw 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."
Navalny had drunk a cup of tea at a Tomsk airport cafe before boarding his flight, Yarmysh said. The Interfax news agency quoted the cafe's owners as saying they were checking CCTV cameras to try to establish what had happened.
Navalny's plane later made an emergency landing in Omsk so that he could be rushed to hospital. Footage posted on social media by a fellow passenger showed a motionless Navalny being stretchered into an ambulance by medics.
S7, the airline he was travelling with, said Navalny had started feeling very ill soon after take-off and that the captain had decided to make an emergency landing at a nearby airport.
It said he had not eaten or drunk anything on board.
The TASS news agency cited the head doctor at a hospital in Omsk confirming that Navalny had been admitted there and saying he was in a serious condition.
Yarmysh drew a parallel with an incident last year in which Navalny suffered an acute allergic reaction that one doctor said could have resulted from poisoning with an unknown chemical.
"Obviously the same has been done to him now," Yarmysh said.
The European Court of Human Rights has ruled that Russia's arrests and detention of Navalny in 2012 and 2014 were politically-motivated and violated his human rights, a ruling Moscow called questionable