Russian opposition leader Alexei Navalny was rushed to hospital after a suspected poisoning on Thursday, August 20, Deutsche Welle reports.

He was unconscious and in intensive care after his airplane made an emergency landing, according to his spokeswoman, Kira Yarmysh. He later fell into a coma, she said.

He started feeling unwell on a flight from the Siberian city of Tomsk to Moscow, she said. The plane landed in Omsk and he was taken to hospital.

"We assume that Alexei was poisoned with something mixed with his tea," she said. "That's the only thing he drank this morning."

She later posted that Alexei was in a coma and had been connected to a ventilator. She said the police had been called to the hospital at their request. Tass news agency reported that he was in a serious condition, citing the head doctor at Omsk Ambulance Hospital number 1.

Medical authorities told Interfax news agency that tests had shown he was poisoned with an unidentified psychodysleptic (hallucinogen).

Navalny spends his time exposing corruption among the Russian elite and has suffered a constant campaign of intimidation and threats. He suffered a similar incident last year where he was also suspected to have been poisoned while in police custody.