Moscow's Tver District Court found Udaltsov guilty of organizing a mass gathering on October 12 in front of the Moscow Mayoral Office without official permission and fined him the equivalent of $50.
Udaltsov, coordinator of the Left Front movement, told RFE/RL that he does not agree with the verdict and plans to appeal.
On October 12, activists in Moscow tried to stage a mass gathering in front of the mayor's office in order to send a message to the soon-to-be-named replacement for Yury Luzhkov as mayor. Sergei Sobyanin, a close aide to Prime Minister Vladimir Putin, was confirmed as mayor on October 21.
Police dispersed the gathering, arresting dozens of people, including Udaltsov and well-known human rights activist Lev Ponomarev.