The doctors are working hard to save his life, says deputy head of the hospital Kalinitenko.
Alexei Navalny was on a flight from the Siberian city of Tomsk to Moscow when he fell ill, which forced the plane to cancel the trip and land in Omsk, where he is now in intensive care. Navalny's staff say his condition is serious.
"We assume that Alexei was poisoned by something mixed in his tea. It was the only thing he drank this morning ", writes the spokesperson Kira Jarmysj on Twitter.
"Doctors say the poison was absorbed quickly due to the hot drink," she continues.
Alexei Navalny, 44, a lawyer, anti-corruption activist and strong Putin critic, was on a campaign trip ahead of September's Russian regional elections when he fell ill. A fellow passenger on the flight from Tomsk writes on social media that Navalny "screamed in pain" during the flight.
Representatives of the Omsk hospital tell reporters on the spot that it has not yet been confirmed whether Navalny was poisoned and that "natural poisoning" is one of several explanations being considered. According to Deputy Hospital Director Anatoly Kalinichenko, Navalny is being cared for on a respirator.
Doctors do not just do everything they can. The doctors are working hard to save his life, Kalinitenko told the press.
Sources tell the Russian news agency Interfax that doctors have tentatively diagnosed Navalny with poisoning from an unidentified hallucinogenic substance.
The Putin critic has been repeatedly detained in recent years, including for organizing government-critical protests, and has been sentenced to several prison terms. He has also recently spoken out in support of protests in Belarus against authoritarian President Alexander Lukashenko.
The Kremlin announces in a statement that it follows developments around Navalny's condition.
"As with all Russian citizens, we wish him a speedy recovery," said spokesman Dmitry Peskov.
He states that there is still no evidence of poisoning.