Russian police said they detained four men who tried to interfere in a sanctioned rally in St. Petersburg on Sunday to support Ukraine's pro-European opposition movement.
One of the protest-crashers, Anatoly Artyukh, was taken into custody after he tried to snatch a Russian flag away from a demonstrator.
Police said Artyukh, the local chair of Orthodox nationalist organization Narodny Sobor and an alleged aide to outspoken religious lawmaker Vitaly Milonov, claimed the protesters had no right to wave the Russian symbol.
The detainees, who police said included another unnamed Milonov aide, were taken to a local police station.
About 50 people attended Sunday's rally to support Ukrainian protesters, the Interior Ministry estimated.
Activists from the Party of Progress, a Russian political opposition faction headed by anti-corruption blogger Alexei Navalny, attended the protest along with members of the opposition December 5 party and the far-right National Democrats party.
Ukraine's capital Kiev has been rocked by anti-government rallies since November, after President Viktor Yanukovych rejected an association agreement with the European Union at the last minute to sign a $15 billion aid package with Russia instead.
The demonstrations initially vented anger at the abandoned trade deal, but quickly turned into a broad anti-government protest movement.