A video circulating on social media Thursday purportedly shows medical workers attending to Alexei Navalny on a plane as he screams in pain after what his allies allege was a poisoning engineered by the Kremlin.

The video, which appears to have been first posted on Instagram Stories by a passenger, shows the medics rushing down the aisle as other passengers look on as a man can be heard moaning in agony off-camera.

"At the start of the flight (Navalny) went to the toilet and he didn't come back," the passenger posted in a message to the Instagram story. "He became very ill and they could barely resuscitate him as he screamed in pain. They are not saying what is wrong with him. Now we have landed in Omsk. An ambulance came."

Navalny's spokeswoman, Kira Yarmysh, tweeted that he had fallen ill during a flight back to Moscow from the Siberian city of Tomsk and was taken to a hospital after the plane made an emergency landing in Omsk

Yarmysh told the Echo Moskvy radio station that he must have consumed poison in tea he drank at an airport cafe before boarding the plane early Thursday. During the flight, Navalny started 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, and has been in a coma and on a ventilator in grave condition since.

Other opposition figures were quick to suggest Kremlin involvement.

"We are sure that the only people that have the capability to target Navalny or myself are Russian security services with definite clearance from Russia's political leadership," Pyotr Verzilov, a member of the protest group Pussy Riot who ended up in intensive care after suspected poisoning in 2018, told The Associated Press. "We believe that Putin definitely is a person who gives that go-ahead in this situation."

An ambulance is parked outside the hospital intensive care unit where Alexei Navalny is hospitalized in Omsk, Russia, Thursday, Aug. 20, 2020. 

An ambulance is parked outside the hospital intensive care unit where Alexei Navalny is hospitalized in Omsk, Russia, Thursday, Aug. 20, 2020.  (AP)

Doctors at Omsk Ambulance Hospital No. 1, where the politician was being treated, remained tight-lipped about his diagnosis, saying only that they were considering a variety of possible theories. Local health officials said they found no indication that Navalny had suffered from a heart attack, stroke or the coronavirus.

Kremlin spokesman Dmitry Peskov said it was necessary to wait for test results showing what caused Navalny's condition, adding the authorities would consider a request to allow Navalny to leave Russia, which has not fully opened its borders after a coronavirus lockdown, for treatment.

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Navalny has been frequently detained by law enforcement and harassed by pro-Kremlin groups for his outspoken opposition to Putin. In 2017, he was attacked by several men who threw antiseptic in his face, damaging an eye.

Last year, Navalny was rushed to a hospital from prison, where he was serving a sentence following an administrative arrest, with what his team said was suspected poisoning.

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Navalny's Foundation for Fighting Corruption has been exposing graft among government officials, including some at the highest level. Last month, he had to shut the foundation after a financially devastating lawsuit from Yevgeny Prigozhin, a businessman with close ties to the Kremlin.

The Associated Press contributed to this report.