Police detained about 55 protesters on Sunday outside the gates to Red Square, which was unexpectedly closed to all visitors and tourists to prevent an anti-Kremlin demonstration.
Opposition activists had called on supporters to walk around the square wearing the white ribbons that have become a symbol of the protest movement against Prime Minister Vladimir Putin and the stifling of democratic politics during his 12 years in power.
When police took the unusual step of closing the vast cobblestone square near the Kremlin, about 300 protesters gathered instead just outside the gates. The meeting place, communicated through social networking sites, was the "zero kilometer," the spot from where all distances from Moscow are measured.
Holding hands to form a circle, the protesters chanted "This is our city," "Russia will be free" and "Russia without Putin."
Some of the protesters then demanded to be allowed onto Red Square and police rounded them up, leading or carrying them onto waiting buses.
Police said about 55 people were detained.
Putin faced unprecedented protests by tens of thousands of people in the months ahead of a March presidential election. Since his victory, the street protests have dwindled and have been routinely broken up by police. Those detained have usually been released by the end of the day.
The protest movement, however, has inspired a rise in civic activism and involvement in local politics.
Hundreds of volunteers from Moscow were observing a second-round mayoral election on Sunday in the city of Yaroslavl, 250 kilometers (150 miles) northeast of the capital, where the opposition candidate was in a runoff with the candidate supported by the local and regional governments.
Putin, who served as president from 2000 to 2008, will return to the Kremlin in May for a third, now six-year term.
Associated Press writer Lynn Berry contributed to this report.