Navalny was on a flight from the Siberian city of Tomsk to Moscow when his plane had to cancel the trip after he became ill, according to Yarmysh.

"We assume that Alexei was poisoned by something mixed in his tea. That was the only thing he drank this morning ", writes Jarmysj on Twitter.

"The doctors say that the poison was quickly absorbed due to the hot drink," Jarmysj continues.

Has been imprisoned before

Navalny is admitted to the intensive care unit of a hospital in the city of Omsk, reports Tass. The airport in Omsk confirms to Tass that an aircraft landed there due to a passenger feeling bad. "An ambulance arrived," said a spokeswoman for the airport, who added, however, that she did not know the identity of the passenger.

Alexei Navalny, a lawyer and anti-corruption activist, has been repeatedly detained in recent years for organizing government-critical protests. He has previously been subjected to physical attacks. In 2017, he suffered chemical burns to one eye when perpetrators threw disinfectants at his face.