(NEW YORK) — Russia’s best-known anti-Kremlin opposition leader Alexey Navalny is in a coma in intensive care after being allegedly poisoned with a toxin slipped into his tea, according to his press secretary.

His press secretary Kira Yarmysh wrote that Navalny fell sick while on a plane returning from Siberia to Minsk on Thursday morning and that the plane made an emergency landing in Omsk where he was rushed to hospital.

Navalny is unconscious in a coma and connected to a ventilator, Yarmysh wrote in a series of tweets, saying his condition is “serious.”

“We assume that Alexey was poisoned with something mixed in tea. It was the only thing that he drank since the morning. The doctors say that the toxin was quicker absorbed by hot liquid,” Yarmysh wrote on Twitter.

She said they had called the police.

The hospital in Omsk confirmed to the state news agency TASS that Navalny is in its toxicology intensive care unit and its chief doctor said he condition was serious but “stable.”

Navalny, 44, is Russia’s most prominent opposition leader who has sought to challenge President Vladimir Putin through anti-corruption investigations and building a grassroots protest movement. His investigations, normally published as videos, have exposed alleged corruption among top Russian officials, including some of members of Putin’s inner circle, attracting millions of views. In recent years, his organization has helped lead some of the largest protests against Putin in Moscow.

Putin’s spokesman, Dmitry Peskov, on Thursday told reporters the Kremlin was aware of Navalny’s hospitalization and wished him “the speediest recovery.”

“Obviously, as with any citizen of our country, we wish him the speediest recovery,” Peskov said in a daily briefing call.

He said that it was still till too soon to say whether Navalny had been poisoned but that if he had an investigation should be opened.

“We are talking about a citizen of the Russian Federation. First we need to wait for the final analyses, which will help the doctors to determine what became the cause of this situation, the loss of consciousness. After that, if poisoning took place and there will be the relevant applications and if various decisions will be taken by law enforcement bodies then there will be an investigation,” Peskov said.

Peskov said the Kremlin saw no reason why Navalny should not be taken abroad for treatment and said it was ready to assist if such a request was made.

A number of Kremlin opponents have fallen victim to violence over the years, also to suspected poisonings. In 2015, Boris Nemtsov, a former prime minister and one of the country’s best-known opposition figures was shot dead on a bridge in front of the Kremlin.

There have been a series of high-profile poisoning incidents involving Kremlin critics recently, including Sergey Skripal, the former Russian double-agent who was poisoned with a nerve agent in the British town of Salisbury in 2018. Another prominent activist, Vladimir Kara-Murza, a member of the pro-democracy group Open Russia, nearly died twice after suffering suspected poisoning in 2015 and again in 2017.

In 2018, Petr Verzilov, a member of the protest group Pussy Riot, was flown to a hospital in Germany after suffering what doctors there said was a near-fatal poisoning with an unknown substance.

Pavel Lebedyev, a man who said he was on the same flight as Navalny posted photos and videos of Navalny sitting and drinking tea in the airport and then another video onboard the plane where a person can be heard moaning in agony.

“He went to the toilet, and for a long time didn’t return,” Lebedyev said in a video posted to his Instagram stories.

Then, he said, he realized the crew were announcing an emergency landing.

“I thought someone’s got ill. I turned my head and I understood that Alexey is just lying there,” Lebedyev said.

Navalny’s spokeswoman, Yarmysh, wrote she was concerned that doctors at the hospital now were facing pressure from authorities, writing that the ward was now full of police officers and alleged doctors had become hesitant about talking with her about Navalny’s diagnosis.

At a news conference, the deputy chief doctor at Omsk’s Urgent Medical Care hospital No.1, Anatoly Kalinchenko, told Russian media that he could not confirm at the moment that Navalny had been poisoned, saying there was “no certainty” of that yet and that it could also be the result of “natural poisoning.”

Russia’s state news agency TASS cited an unnamed law enforcement official saying that for now police are not treating Navalny’s hospitalization as a deliberate poisoning and instead implied the opposition leader could have somehow have taken a poisonous substance himself.

“It’s not excluded that he swallowed or took it himself,” the source was quoted by TASS.

Photos posted by Navalny’s companions showed the opposition leader smiling and looking well on the airport bus before take-off. Within a couple of hours, he was unconscious and in a serious condition.

Last year, Navalny was hospitalized with what his team at the time said was poisoning, after he suffered severe inflammation of his face while serving a short jail sentence for protesting. At the time, authorities said he had suffered an “allergic reaction.”

Navalny has been arrested and jailed dozens of times in the past decade and faced attacks and threats from pro-Kremlin activists. During one assault he almost lost his sight after green anti-septic was thrown in his face. He has become the best-known of Russia’s anti-Kremlin opposition leaders and his group has built up a national following.

Last summer, when several opposition activists allied with Navalny were barred from running in Moscow’s local elections, it triggered weeks of protests, the largest and most persistent the Russian capital has seen in around a decade.

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