Mr Navalny was flying to Moscow from Tomsk and was diverted to Omsk after he fell ill.
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. Read more >>