The 44-year-old Navalny, a staunch critic of Russian President Vladimir Putin, fell ill on a flight from the Siberian city of Tomsk to Moscow, Yarmysh said.
“The plane made an urgent landing in Omsk. Alexei has toxic poisoning. Right now we are going to the hospital,” Yarmysh tweeted.
Russia’s TASS news agency reported that the Omsk Emergency Hospital had confirmed Navalny’s admission and that his condition was serious.
“We suspect that Alexei was poisoned by something mixed into his tea. It was the only thing he drank since morning,” Yarmysh wrote in another tweet. “Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious.”
Yarmysh said Navalny was in a coma and placed on a ventilator in an intensive care unit. Anatoliy Kalinichenko, the Omsk doctor treating Navalny, told reporters that he is in stable but serious condition, according to a video tweeted by Yarmysh. Kalinichenko declined to confirm whether Navalny was in a coma or give details about his diagnosis.
Photos and video published online captured the sequence of events.
On Instagram, a man named Pavel Lebedev, who was on the same flight as Navalny, published a picture of the opposition leader sipping tea at the Tomsk airport before takeoff.
In a video Lebedev filmed aboard the flight that was also published on the Mash Telegram channel, a man said to be Navalny can be heard screaming. Lebedev wrote on Instagram that Navalny went to the plane’s bathroom after takeoff and never came out.
A second video published by Mash appears to shows an immobile Navalny being loaded onto a gurney after the plane made touched down in Omsk.
A local news outlet published a video of him being put into an ambulance on the tarmac of the Omsk airport.
Navalny, a lawyer and prominent anti-corruption campaigner, has been a thorn in the side of Putin and the face of the Russian opposition for a decade. In that time he has been attacked, beaten, and jailed for various infractions, including calling for unauthorized protests and embezzlement. He has denounced all charges against him as political.
In July 2019, while serving a 30-day jail term for organizing unauthorized protests, he fell ill and was diagnosed with “contact dermatitis.” But he said he thought he had been poisoned. His personal doctor also said he may have been exposed to an unknown “toxic agent.”
In June this year, Navalny described a controversial vote on changes to Russia’s constitution that was pushed by Putin as a “coup” and a “violation of the constitution.” The changes, which were approved in a national referendum, allow Putin to serve another two terms in office and lead the country until 2036.