Moscow, Aug 20 (IANS): Russian opposition leader Alexei Navalny was unconscious in a hospital after suffering from suspected poisoning, his spokeswoman said on Thursday.

Kira Yarmysh, the press secretary for the Anti-Corruption Foundation, which Navalny founded in 2011, tweeted: "This morning Navalny was returning to Moscow from Tomsk. During the flight, he felt ill. The plane made an urgent landing in Omsk. Alexei has toxic poisoning."

"We suspect that Alexei was poisoned by something mixed into (his) tea. It was the only thing he drank since morning.

"Doctors are saying that the toxic agent absorbed faster through the hot liquid. Right now Alexei is unconscious."

Yarmysh later tweeted that Navalny, the 44-year-old staunch critic of President Vladimir Putin, was on a ventilator and in a coma, and that the Omsk Emergency Hospital was now full of police officers, the BBC reported.

She said doctors were initially ready to share any information but were now saying the toxicology tests were delayed and were "clearly playing for time, and not saying what they know".

Navalny is in the intensive care unit.

The hospital's chief physician Alexander Murakhovsky earlier told TASS News Agency that he was in serious condition, adding that doctors were yet to diagnose his case.

Addressing reporters, Anatoly Kalinichenko, deputy chief physician, said: "There are diagnoses that have been ruled out or are in the process of confirmation. (Doctors) are holding consultations. We will decide on the diagnosis later in the day.

"Symptomatic treatment (is currently being conducted)."

Kalinichenko added it was not yet certain if Navalny was poisoned.

Navaln has made a name for himself by exposing official corruption, labelling Putin's United Russia as "the party of crooks and thieves", and has served several jail terms.