Udaltsov, leader of the opposition Left Front movement, arrived Friday at a prosecutor's office in Moscow for questioning related to alleged anti-state activity. Violetta Volkova, his lawyer, said her client is "afraid of nothing," state-run news agency RIA Novosti reports.
The opposition leader last week was called on to arrive with financial documents related to his movement's activity.
Udaltsov is under investigation for allegedly discussing revolutionary ambitions with political leaders in Georgia. The investigation was launched after a low-quality video surfaced that showed him meeting with Georgian leaders. He said the video was a forgery.
Leonid Razvozzhayev, a close ally of Udaltsov's, is also in custody. He said he was abused by Russian authorities after getting arrested in Ukraine on charges of supporting the ouster of Russian President Vladimir Putin.
Putin's election to a third non-consecutive term in May sparked widespread protests in Russia.
Ray Liotta sues skin care company over use of likeness
NBC reportedly holds celebs hostage to Jimmy Fallon's show