Police detained four people heading for the square from a nearby subway station, and later picked people out of the crowd at the gates and pushed them into buses. A police spokesman said about 55 people had been detained near Red Square "for attempting to hold an unsanctioned demonstration."
Opposition leaders say the government violates Russians' constitutional right to free assembly by requiring permission from local authorities for street demonstrations. Police often disperse unsanctioned rallies and detain protesters.
"There are no freedoms, no rights, and Putin is always being shown on all the TV channels," said Inna Bachina, 48, director of a metal products wholesaler, standing outside the Red Square gates with her daughter. "I do not consider Putin my president."
Opposition activists have held several small demonstrations without official permission since the disputed December election won by Putin's ruling party, while other protests have been held with permission.
Putin, president from 2000-2008 and now prime minister, won a new presidential term on March 4 and will be inaugurated on May 7. He could seek re-election in six years.
Tens of thousands of people dismayed by that prospect and angered by suspicions of widespread fraud in the parliamentary election turned out for rallies in recent months - the biggest opposition protests of Putin's rule.
He and President Dmitry Medvedev have introduced limited political reforms, including legislation to let more groups register as parties and contest elections, but some foes fear a crackdown on street protests and other opposition activity.
On Saturday, riot police detained about 60 people in Moscow and 15 in St. Petersburg, dispersing protesters shouting "Russia without Putin" and "Freedom of Assembly: Always and Everywhere!"
Activists said the authorities would have no reason to detain people simply strolling on Red Square with white ribbons. Bachina said she believed the square was closed because "the authorities are frightened."
Police officers gave various reasons for the closure of the square, one saying it was shut for repair work, while another cited an unspecified event due to be held there. The Federal Guard Service, responsible for security in and around the Kremlin, did not answer phone calls.