Footage circulating on social media purportedly shows medical workers on a plane attending to Alexei Navalny as he screams in pain after an alleged poisoning engineered by the Kremlin.
The video, which appears to have been first posted on Instagram Stories by a passenger, shows medics rushing down the aisle and other passengers staring as a man is heard yelling in agony off-camera.
A senior doctor at the hospital where Russian opposition leader Navalny is critically ill in hospital from suspected poisoning said medics were "doing everything possible" to save his life.
Navalny, an anti-corruption campaigner and one of Russian President Vladimir Putin's key political rivals, is in intensive care in a hospital in the Siberian city of Omsk after he was poisoned with a toxin mixed into his tea, according to his press secretary Kira Yarmysh.
Yarmysh said the 44-year-old was in a coma on a ventilator.
She also tweeted his wife Yulia Navalnaya has rushed to the hospital in Omsk along with his doctor from Moscow, but neither are allowed in the intensive care unit to see him.
She pointed the finger at Putin, saying: "Whether or not he gave the order personally, the blame lies with him."
It comes as a separate video of Nalvany being taken off the plane in a stretcher circulates online, and the cafe's owners say they can no longer find the waiter who served him.
The alleged poisoning comes two years after the novichok attack on ex-Russian spy Sergei Skripal, 67, and his 34-year-old daughter Yulia, by Putin-backed GRU agents in the UK.
It's also 14 years after another Putin critic, Alexander Litvinenko, died after having his tea poisoned with radioactive polonium-210 at a plush hotel in London.
The deputy head of the emergency hospital, Anatoly Kalinichenko, said staff were working hard to save Navalny's life.
"Doctors aren't just doing everything possible. The doctors are really working now on saving his life," Kalinichenko told journalists, according to AFP.
The Kremlin said it was aware of Navalny's illness and wished him well but there was no evidence yet to suggest he had been poisoned.
"We are reading this information. We know that he is in serious condition. Like with any citizen of the Russian Federation we wish him a speedy recovery," Kremlin spokesman Dmitry Peskov told journalists, AFP reported.
"As far as we know there are not yet any test results. All assumptions are only assumptions," he added.
"Whether it was a poisoning needs to be confirmed by a laboratory."
Peskov said "there will be an investigation" should tests confirm Navalny had been poisoned.
According to tweets from his press secretary, Navalny fell ill as he was flying back to Moscow from Tomsk, Siberia, where he was meeting opposition candidates.
Yarmysh said he had been "absolutely fine" on the way to the airport on Thursday morning, local time, and drank only black tea while waiting for his flight.
"Straight after takeoff he quite quickly lost consciousness," she said.
"We think that Alexei was poisoned with something mixed in his tea. That was the only thing he drank in the morning."
Yarmysh told the Echo of Moscow radio station that she was "sure it was intentional poisoning".
A passenger on the flight to Moscow said Navalny was "screaming in pain".
"At the start of the flight he went to the toilet and didn't come back. He started feeling really sick," passenger Pavel Lebedev said, according to the BBC.
"They struggled to bring him round and he was screaming in pain."
Navalny, who founded the Anti-Corruption Foundation in 2011, is a staunch critic of Putin and recently slammed Russia's constitutional reforms that allowed Putin to serve another two terms in office — as a "coup" and "violation of the constitution".
He has been imprisoned numerous times for supporting illegal protests, including a 30-day sentence in July last year after he called for opposition rallies in Moscow.
His bid to enter the 2018 presidential race was barred because of fraud convictions he said were politically motivated.