At least 250 people were detained by police at a protest in downtown Moscow, where flare-type fireworks were thrown at a group of pro-Kremlin youth, said city police spokesman Maxim Kolosvetov.
Russian news agencies reported about 200 were arrested at a similar attempt to hold an unsanctioned rally in St. Petersburg and another 25 protesters were arrested in the southern city of Rostov-on-Don.
It was the second consecutive night of large protests in Moscow and St. Petersburg, an unusually sustained show of indignation that came after widespread reports of vote fraud in Sunday's parliamentary election. Russian police routinely crack down hard on unauthorized rallies and protesters generally take time to regroup for a new attempt.
According to preliminary results,Putin's dominant United Russia party lost a large share of the seats it had held in the State Duma but still kept a majority of seats.
Opponents say even that reduced presence came because of vote fraud. Local and international election observers reported widespread ballot-stuffing and irregularities in the vote count.
The protesters appear to be both angered by the reported fraud and energized by the vote's show of declining support for Putin and his party, which have strongly overshadowed all other political forces in Russia for a dozen years.
But pro-Kremlin supporters also put on two large rallies in Moscow, attracting thousands and showing vehement divisions in Russian society.
