Alexei Navalny, Vladimir Putin's arch-rival and critic, has been rushed unconscious to hospital after apparently being poisoned.

The lawyer and anti-corruption campaigner fell ill on a flight from Tomsk to Moscow, forcing the plane to make an emergency landing in the Siberian city of Omsk.

Video shows the Navalny being stretchered from the aircraft into a waiting ambulance before being rushed to intensive care.

He is now in a coma and hooked up to a ventilator, with doctors saying his condition is 'grave', press secretary Kira Yarmysh revealed.