Russian opposition figure Alexei Navalny is unconscious in hospital suffering from suspected poisoning, his spokeswoman has said.
The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in Omsk, where doctors said he was in a coma and they were trying to save his life.
His team suspects something was put in his tea at an airport cafe.
The Kremlin said that it wished Mr Navalny a “speedy recovery”.
Mr Navalny, 44, has for years been among President Vladimir Putin’s staunchest critics.
In June he described a vote on constitutional reforms as a “coup” and a “violation of the constitution”. The reforms allow Mr Putin to serve another two terms in office, after the four terms he has already had.