Russian opposition leader Alexei Navalny is unconscious and has been placed on a ventilator in a hospital in Siberia after falling ill from suspected poisoning, his spokesperson said.
The outspoken Kremlin critic fell ill during a flight and the plane made an emergency landing in Omsk, Kira Yarmysh said, adding that they suspected something had been mixed into his tea.
"We assume that Alexei was poisoned with something mixed into the tea. It was the only thing that he drank in the morning. Doctors say the toxin was absorbed faster through the hot liquid," said Yarmysh.
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 remains unconscious and is now connected to a ventilator, Yarmysh added.
Navalny, who has campaigned against Putin’s rule for years, was travelling through several cities in Siberia to back candidates he supports in local elections involving 40 million voters next month.
In June he described a vote on constitutional reforms as a "coup" and a "violation of the constitution". The reforms allow Mr Putin to serve another two terms in office.
Russian opposition leaders have been targeted with violence in the past. In 2015, opposition leader and former deputy prime minister Boris Nemtsov was shot four times and killed within sight of the Kremlin. Five men from Chechnya were jailed in the attack, although his family believes that those who ordered the killing were never caught.
Mr Navalny made a name for himself by exposing official corruption and labelling Mr Putin's United Russia as "the party of crooks and thieves".
Mr Navalny was briefly jailed in July 2013 on embezzlement charges but denounced the sentence as political.
He attempted to stand in the 2018 presidential race but was barred because of previous fraud convictions in a case he again said was politically motivated.
Navalny in recent weeks has used the Belarus protests against its president, Alexander Lukashenko, to try to persuade Russians to back candidates he supports in next month’s local elections.
In a recent appearance on his YouTube channel, Navalny spoke excitedly of how successful strikes by key workers in Belarus had forced authorities to start engaging with protesters.
Activists believe they may face a Belarusian scenario when Putin comes up for re-election in 2024 after he successfully got the constitution changed to allow him to run again for president twice.