Udaltsov, who has supported the war in Ukraine, had written on the Telegram messenger app earlier on Thursday morning that police were breaking down his front door.
Udaltsov's lawyer provided no further information about the charges levelled at her client.
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 2011-2012 protests in Moscow after allegations of widespread electoral falsification, and was allied at the time with now-jailed Kremlin critic Alexei Navalny. Udaltsov was jailed in 2014 for organising protests and released in 2017.
While remaining critical of Russian President Vladimir 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 Timothy Heritage)
