State news agency RIA said Udaltsov, a long-time critic of Russian President Vladimir Putin who nevertheless backs the war in Ukraine, had told reporters that the charges were related to his support for members of a Marxist group in the Volga river city of Ufa who were charged with terrorism in 2022.
Under Russian law, the charges carry between five and seven years in prison.
Udaltsov is the leader of the Left Front, a coalition of Russian left-wing groups affiliated with Russia's Communist Party. His wife Anastasia serves as a communist member of Russia's parliament.
Udaltsov rose to prominence during 2011-12 protests in Moscow after allegations of widespread electoral falsification, and was allied at the time with jailed Kremlin critic Alexei Navalny. Udaltsov was imprisoned in 2014 for organising protests and released in 2017.
While remaining critical of Putin, Udaltsov has, since the start of the conflict in Ukraine, broken with former opposition allies by supporting the Kremlin's military campaign.
(Reporting by Felix Light; editing by Barbara Lewis)
