At least three activists were arrested after pitching a tent near Lenin's Mausoleum on Red Square during a flash mob-style opposition protest Sunday.
Prominent environmental campaigner Yevgenia Chirikova and two others were taken to a nearby police station after being detained, and dozens of activists streamed toward the station after the arrests to "liberate" Chirikova, Interfax reported.
Police linked arms to block protesters from entering the station, but eventually allowed three people to enter to check on the detainees.
An Interior Ministry spokesman told Interfax that the arrested activists would be charged with disorderly conduct, which carries a maximum penalty of 15 days in jail. Chirikova said on Twitter that she would remain in custody until a court appearance scheduled for Monday.
The "White Square" flash mob was attended by participants wearing white clothing, white scarves and white ribbons. One young woman huddled under a white blanket with a sign declaring her to be the "Ghost of the Constitution."
Opposition leaders Sergei Udaltsov and Boris Nemtsov made an appearance, handing out fliers announcing the People's March to be held May 6, the eve of the inauguration of President-elect Vladimir Putin.
About 50 pro-Kremlin youth activists dressed in military outfits also came to the square, singing war songs and reciting patriotic poems.
Leading up to the event, Red Square was surrounded by metal detectors, and at least 10 police buses were spotted nearby.
A second opposition event called "White Metro" was also held Sunday. Dozens of activists converged on metro station Ploshchad Revolyutsii and its two connecting stations to leave white stickers, ribbons, roses and balloons.
Police detained several demonstrators attempting to post fliers with information about the People's March event, as well another young man wearing a Guy Fawkes mask, a reference to the mask worn by a revolutionary in the 2005 film V for Vendetta, Interfax reported.
Police called for demonstrators to leave using a loudspeaker, and at one point began attempting to force crowds to leave the metro station by forming a chain and pushing activists towards escalators, a tactic used at previous protest events.
Last week, 85 were detained in a similar event after Red Square was closed "for repairs." Protesters had called on protesters to "turn Red Square white."