Russian authorities last week limited the movement of Udaltsov after he was questioned for an alleged plot to force Russian President Vladimir Putin from office.
Udaltsov, leader of the opposition Left Front movement, said he was 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, a video he said was a forgery.
Vladimir Markin, a spokesman for a Russian investigative committee, said Leonid Razvozzhayev, a close ally of Udaltsov's, is in custody, reports RIA Novosti. The spokesman said Razvozzhayev confessed to assisting Udaltsov, adding the claims "will be thoroughly investigated."
Last week, Alexandra Makovskaya, a spokeswoman for the U.N. High Commissioner for Refugees in Ukraine, told the BBC that Razvozzhayev had disappeared from Kiev following a meeting with U.N. officials. The BBC reports his allies said he was kidnapped by Russian authorities.
Opposition to Putin has grown since he secured a non-consecutive third-term in office in an early 2012 election viewed by some monitoring groups as fraudulent.