Russian anti-graft fighter Alexei Navalny on ventilator after suspected poisoning

Russia’s most prominent opposition leader and anti-corruption campaigner, Alexei Navalny, was reported Thursday to be critically ill after an apparent poisoning while on a flight from Siberia to Moscow.

A spokesperson for Navalny, 44, said the plane made an emergency landing after he became unconscious after drinking tea.

He was taken to a hospital in Siberia and placed on a ventilator to help him breathe.

Navalny has called Russian President Vladimir Putin “the Tsar of corruption” and labeled the ruling United Russia party “the party of crooks and thieves.”

He recently campaigned against constitutional amendments that will allow President Putin to remain in power until 2036. The amendments were passed last month.

He trained as a lawyer. In 2010, he published what he said was evidence of a $4 billion fraud at Transneft, a state-owned firm with a monopoly on transporting oil produced in Russia.

His YouTube channel has nearly four million subscribers. His earlier anti-corruption blog was one of Russia’s most popular websites.

He has led mass protests in Moscow and other cities against the Kremlin and been arrested multiple times.

In 2017, a Russian court convicted him of embezzlement. The conviction resulted in his being barred from running for political office. A judge suspended his five-year jail sentence.

He ran for mayor of Moscow in 2013. He lost to the incumbent, Sergei Sobyanin, a Putin crony. But he won nearly 30 percent of the vote, surprising the Kremlin.

At a news conference Thursday, a doctor at the hospital where Navalny is being treated said poisoning is being “looked at as one of the possible reasons for the deterioration of his condition.”

Navalny had been in Siberia this week to support candidates running for local office.