Russian opposition figure Alexei Navalny is unconscious in hospital with suspected poisoning, his spokeswoman has said.
The anti-corruption campaigner 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.
Mr Navalny, 44, is a staunch critic of President Vladimir Putin.
Video shows the unconscious Navalny being stretchered from the aircraft into a waiting ambulance before being rushed to intensive care.
The 44-year-old was pictured drinking the tea at the Vienna Cafe at Tomsk Airport.
The manager of the cafe said the staff member who served him the drink now cannot be found.
The cafe has been closed and an investigation was underway, the manager said.
He is said to be in grave condition.
Medics have given a preliminary diagnosis of ‘toxic poisoning’, but no test results have yet come back.