Alexei Navalny, an anti-corruption activist and vocal critic of Russian President Vladimir Putin, is in a critical condition in the intensive care unit of a Siberian hospital after being "poisoned with a toxin," according to his press secretary.

Navalny, 44, was reportedly rushed in an unconscious state to the hospital on Wednesday night after ingesting a poisonous material believed to have been mixed in his morning tea, Kira Yarmysh tweeted on Thursday.

Navalny Fell Sick on Flight from Moscow to Tomsk

Alexei Navalny
Alexei Navalny Twitter

"This morning Navalny was returning to Moscow from Tomsk. On the flight he started feeling ill. The plane made an emergency landing in Omsk. Alexei has been poisoned with a toxin. Right now we're in an ambulance on the way to the hospital," read the translated tweet.

"At the start of the flight he went to the toilet and didn't come back," said Pavel Lebedev, a passenger on the flight "He started feeling really sick. They struggled to bring him round and he was screaming in pain."

Yarmysh added that Navalny is believed to have been poisoned with something mixed in his tea as it was the only thing he consumed on Wednesday morning. According to doctors, the toxin was absorbed much more quickly as the beverage was hot. In a follow-up tweet, she confirmed that Navalny is in a coma and on ventilator support.

Doctors Being 'Evasive'

In a series of tweets posted a short while ago, Yarmysh claimed the doctors promised the test results within a duration of two hours but are now delaying the results and refusing to share any information with Navalny's attending physician.

"The evasive reaction of doctors only confirms that this is poisoning," she wrote. "Two hours ago, they were ready to share any information, but now they are clearly playing for time and do not say what they know.

The press secretary noted that the ICU is crowded with police officers and at one point one of the doctors who saw her in the distance in the corridor told an officer that "some things are confidential" before taking the officer to another room.

A visibly distressed doctor was seen addressing the media in a press briefing claiming that they are unsure if poisoning was the main cause behind Navalny's condition.

Kremlin Says He Was 'Poisoned' by Anti-Hangover Pill

According to Yarmysh, the Kremlin's Telegram channels are claiming that Navalny ate or drank something himself. The channels are allegedly trying to push the story that Navalny consumed a lot of alcohol in the previous night and took a hangover pill on the morning of his flight, which provoked the poisoning.

However, Yarmysh rejected the claims as "complete nonsense" and clarified that Navalny did not drink the previous night and did not consume any pills the morning of his hospitalization.

Second Assassination Attempt?

Navalny made a name for himself by exposing official corruption, labeling Putin's United Russia as "the party of crooks and thieves." In June he described a vote on constitutional reforms, which allow Putin to serve two more terms in office, as a "coup" and a "violation of the constitution."

His activism and criticism of the Kremlin government has led to him being targeted by authorities in raids and has even been jailed several times.

While serving a 30-day jail sentence in July last year, Navalny fell ill. A jail staffer told his wife, who had arrived for a visit, that an ambulance had rushed him to the hospital with "some kind of strong allergic reaction" that caused his eyes and face to turn red and swell up.

However, he said he had never had any acute allergic reactions and his own doctor suggested he might have been exposed to "some toxic agent."

Putin Silencing His Critics?

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This isn't the first time Putin has been accused of "silencing" critics with poison. In 2009, a Russian lawyer Sergei Magnitsky was poisoned in prison, and some believe it's because he had uncovered a massive government-linked fraud scheme that threatened top officials.

In 2015, the Kremlin government was accused of poisoning former Russian military officer and double agent for the UK's intelligence services Sergei Skripal and his daughter in England with a Novichok nerve agent. After the attack, Putin even went on record to call Skripal a "traitor."