Navalny, 44, started feeling unwell while on a return flight to Moscow from the Siberian city of Tomsk, his spokeswoman Kira Yarmysh, said on Twitter. The plane later made an urgent landing in Omsk, she added.
He only drank black tea in an airport cafe before takeoff, Yarmysh told Russian radio station Echo of Moscow.
“We assume that Alexey was poisoned with something mixed into the tea. It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid,” Yarmysh tweeted.
Navalny remains unconscious and is now connected to a ventilator, Yarmysh said.
Loud groaning can be heard in video footage apparently filmed on the flight taken by Navalny, which was shared on the Baza Telegram channel. More video apparently filmed through the airplane window shows an immobile man being taken by wheeled stretcher to a waiting ambulance.
Navalny has been admitted to the acute poisoning unit of Omsk emergency hospital No. 1 and is in a “serious condition,” hospital head physician Alexander Murakhovsky said, according to Russian state news agency TASS.
The hospital’s deputy head physician, Anatoly Kalinichenko, speaking to local journalists at a makeshift news conference, later confirmed that Navalny was still in the hospital in a serious condition. He was on a ventilator but was stable, the physician said.
Asked by a journalist if Navalny had been poisoned, Kalinichenko said: “Naturally, poisoning is considered as one of the possible reasons for the deterioration of his state. But apart from this, this could be a number of conditions that started acutely and led to the same clinical reactions. We are working on all of them: excluding, confirming.”
Kalinichenko said he believed doctors would have a diagnosis later Thursday. In the meantime, Navalny’s symptoms are being treated, he said.
Yarmysh tweeted a video of the news conference, saying the deputy head physician had confirmed what was already known. “He said the same: stable serious condition, coma, ventilator. Does not say anything about the diagnosis or whether there is a threat to life,” she said.
In an earlier tweet, Yarmysh said the intensive care unit was full of police officers.
“They try to get an explanation from the doctor. The doctor saw me in the distance in the corridor, said that ‘some things are confidential’ and took the police to another room,” Yarmysh said.
“The evasive reaction of doctors only confirms that this is poisoning,” Yarmysh added. “Just two hours ago, they were ready to share any information, and now they are clearly biding for time and are not saying what they know.”
Health ‘sharply deteriorated’
More details are emerging of the events leading up to Navalny’s hospitalization.
Yarmysh told Russian media outlet Mediazona that Navalny had shown no signs of illness until after they had taken off from Tomsk.
“He said that he was not feeling well and asked me for a napkin, he had perspiration,” Yarmysh told Echo. “He asked me to talk to him because he wanted to concentrate on the sound of the voice. I talked to him, after which a trolley with water came up to us — I asked if water would help him; he said no. Then he went to the toilet, after which he lost consciousness.”
S7 Airlines told TASS that the opposition leader had not eaten or drunk anything during the flight.
“Soon after the takeoff of flight S7 2614 Tomsk-Moscow, the state of health of one of the passengers, Alexey Navalny, sharply deteriorated…. While on board, Alexey did not eat or drink anything,” the company said.
According to S7, the crew “worked quickly and strictly in accordance with the procedures.” The flight attendants immediately reported the incident to the aircraft commander who landed the airliner at the nearest airport.
After refueling, the plane went on to Moscow but two passengers who were flying with Navalny stayed in Omsk, TASS said.
Lawyers representing Navalny’s Anti-Corruption Fund (FBK) will submit an application to Russia’s Investigation Committee demanding that it open a criminal investigation into his alleged poisoning, FBK lawyer Vyacheslav Gimadi wrote on Twitter.
Gimadi said the application would be filed under Article 277 of the Criminal Code of the Russian Federation (“Encroachment on the life of a public figure or statesman”).
“There is no doubt that Navalny was poisoned for his political position and activities,” Gimadi said.
He said that despite never suffering from allergies before, he woke up in a detention facility with a dangerous swelling of his face and eyelids. After receiving medical assistance, he was sent back to detention.
Doctors did not find any signs of poisoning after doing analysis on the opposition leader, TASS reported last year.
This story has been updated to correct the spelling of Navalny’s spokesperson.
CNN’s Zahra Ullah and Anna Chernova reported from Moscow. Mary Ilyushina and Darya Tarasova contributed to this report.