MOSCOW (Reuters) - Russian police on Thursday searched the Moscow offices of a pro-democracy movement founded by Kremlin critic Mikhail Khodorkovsky which is calling for big anti-government protests on Saturday, activists said.

Photographs posted on social media showed riot police outside the Moscow offices of the Open Russia Foundation. Maria Baronova, an activist, said on social media that police had confiscated 100,000 flyers promoting Saturday’s protests.

Protest organizers want to use the event to put pressure on Vladimir Putin, who is expected to run for what would be a fourth presidential term next year, to leave politics.

“These (police) actions are aimed at cancelling Saturday’s protests,” a foundation source told Reuters. The police could not be immediately reached for comment.

The searches came a day after the office of the General Prosecutor said it had decided that the activity of the foundation’s British arm was “undesirable.”

Before its offices were searched, the Russian branch had shrugged off the move, saying it was not affected by what amounted to a ban on the British arm’s activities in Russia.

But a second source at the foundation told Reuters on Thursday that the police were seeking to establish and document links between the foundation’s Russian and British arms to make sure they could ban both.

The Kremlin said earlier on Thursday that Saturday’s planned protest was illegal and that police would deal with anyone who showed up accordingly.

In a separate development, an unknown assailant on Thursday threw green liquid at the face of Russian opposition politician Alexei Navalny, who hopes to run for president next year.

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. Navalny suffered a similar attack last month.