The 11 demonstrators detained Monday while picketing the Ukrainian Embassy in Moscow have been released, protest participant and co-leader of the RPR-Parnas party Boris Nemtsov said.
"All participants in today's demonstration at the Ukrainian embassy are free," Nemtsov wrote on his Twitter blog Monday evening.
Police detained the protesters for holding an unsanctioned rally in support of ongoing mass protests in Ukraine, which were sparked by Ukrainian President Viktor Yanukovych's decision last week to suspend a planned association agreement with the EU.
"We support Ukraine's course toward European integration. Both Ukraine and Russia are Europe. In supporting Ukraine we support ourselves too," Nemtsov told Interfax.
Nemtsov, a deputy in the Yaroslavl Regional Duma, said that he intends to challenge his arrest in court.
"They detained me illegally. In accordance with federal law concerning the status of deputies, including regional deputies, the authorization of the General Prosecutor's Office was needed for my detention," Nemtsov said.
Shortly after his own release, Nemtsov said charges of disturbing the public peace were being formalized against the other protesters.
If found guilty, they could be fined up to 30,000 rubles ($900) or sentenced to up to 50 hours of compulsory labor.
Nemtsov has a history of involvement with the Ukrainian political opposition, having spoken at rallies during the Orange Revolution of late 2004 and early 2005.
When Nemtsov's friend Viktor Yushchenko defeated Yanukovych in the repeat runoff election that followed, he appointed Nemtsov as an adviser to help boost Russian investment and improve Russian-Ukrainian relations.
Nemtsov said Monday that he does not intend to take part in the current protests, however.
While the Orange Revolution was inspired by the falsification of election results, which is a matter of international law, the question of association with the EU is "strictly domestic," the politician told RIA Novosti.