MOSCOW — Russian opposition leader Alexei Navalny is in a coma and on a ventilator in a hospital after falling ill from suspected poisoning, his spokeswoman Kira Yarmysh said on Thursday.

"Alexei has been poisoned, intoxicated," and "is now in intensive care," the spokeswoman said in a tweet.

The anti-corruption campaigner fell ill during a flight and the plane made an emergency landing in the Siberian city of Omsk, where doctors said he is in a coma and they are trying to save his life, according to the spokeswoman.

Yarmysh told the Echo Moskvy radio station that Navalny was sweating and asked her to talk to him so that he could “focus on the sound of a voice." He then went to the bathroom and lost consciousness.

Meanwhile, S7 airlines issued a statement, saying: "Soon after take-off of the S7 2614 Tomsk — Moscow flight, the well-being of one of the passengers, Alexei Navalny, sharply deteriorated. The crew worked quickly and strictly in accordance with the procedures. The flight attendants immediately reported this to the captain, who made the decision to land at the nearest airport — in Omsk. Onboard Alexei did not eat or drink anything."

“The ambulance took Alexey straight to intensive care. Two passengers flying with him also stayed in Omsk. After refueling the aircraft, the crew continued their flight to Moscow."

Navalny team suspects something was put in his tea at an airport cafe.

The Kremlin said that it wished Navalny a "speedy recovery".

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 Putin to serve another two terms in office, after the four terms he has already had. — Agencies