Alexei Navalny, a Russian opposition activist and one of the most prominent critics of Russian President Vladimir Putin, was rushed to the hospital Thursday morning after apparently being poisoned.

He is said to be in critical condition and on respiratory support after the plane he was on was forced to make an emergency landing in Omsk in southwestern Siberia.

“We believe that Alexei was poisoned with something that was mixed into his cup of tea, it’s the only thing he drank today,” his spokeswoman said.

Navalny has been jailed multiple times in recent years for his anti-Putin and anti-corruption activities. This article first appeared in Israel Hayom.