Russian opposition politician Alexei Navalny, a fierce critic of President Vladimir Putin and Moscow’s political elite, is unconscious and undergoing treatment at a Siberian hospital after being poisoned, his spokeswoman said.

He started feeling ill after he had tea early Thursday before a flight to Moscow from the Siberian city of Tomsk, Kira Yarmysh wrote on Twitter. The plane made an emergency landing in the nearby city of Omsk, where Navalny was admitted to the hospital, she added.

“We assume that Alexei was poisoned with something mixed in his tea; it was the only thing he drank all morning”, - Yarmysh said. “Now Alexei is unconscious”.

Navalny was hooked up to artificial ventilation at the hospital and was in a critical condition, she said, adding that his supporters had called the police.