Masked police have raided the Moscow offices of a civil society organization funded by former oil tycoon Mikhail Khodorkovsky.

Police said they suspected the group, Open Russia, was preparing banners, leaflets and posters that contained calls for "extremist activity" and planned to distribute them at an opposition protest Sunday.

Khodorkovsky spokeswoman Olga Pispanen said on Facebook that the group was not planning to take part in Sunday's protest and noted that in any case it has been canceled.

Open Russia posted photographs on Twitter of the raid, which took place Thursday during President Vladimir Putin's call-in show.

Khodorkovsky spent 10 years in prison on charges widely seen as punishment for challenging Putin's power. Since his release in late 2013, Khodorkovsky has become a strong opposition voice from exile in Switzerland.