Russian security forces violently dispersed opposition rallies in Moscow and St Petersburg on Saturday, taking into custody former deputy prime minister Boris Nemtsov and dozens other protestors, the broadcaster Echo of Moscow reported.
Some 100 people were reportedly arrested in both cities, dpa reported.
The head of the leading Russian human rights organization Helsinki Group, Lyudmila Alexeyeva, condemned the police brutality on site in Moscow.
Civil rights activists and opposition members had demanded to exercise their right to freedom of assembly.
The demonstration for democracy is usually held on the 31st of the month in a bid to uphold the right, which is guaranteed by Article 31 of the constitution.
But as they have in the past, Russian authorities banned the protest, with the Interior Ministry warning in advance that it would block the event.
That didn't stop some 500 people from gathering on Moscow's Victory Square, according to the opposition website kasparov.ru.
The protesters chanted "Away with the police state," "Shame, police," and "Freedom!" Many also called for the resignation of Prime Minister Vladimir Putin.
Hundreds of police officers were deployed to stop the rally.
Among those arrested were also the leader and several members of a citizens' group campaigning for the protection of a forest in nearby Khimki, which is threatened by the construction of a billion-dollar new highway to St Petersburg.
The fight over the forest has been at the root of unprecedented uproar in Russia over the past weeks.