Moscow: An unknown assailant splashed Russian opposition leader Alexei Navalny on Monday with a green liquid known to be hard to wash off as the vocal Kremlin critic opened his campaign headquarters in the Siberian town of Barnaul.

A video filmed by a Navalny supporter shows the anti-corruption activist walking along a street in Barnaul, more than 3000 kms from Moscow, and shaking hands with residents when a man approaches him and throws the liquid at him.

The green solution, known as "zelyonka", is sold as an antiseptic in Russian pharmacies and while not harmful, it is known for being hard to wash off. Other opposition figures have been splashed with the liquid previously.

Navalny is touring Russia ahead of the 2018 presidential election in which he will challenge incumbent Vladimir Putin.