The unsanctioned protest was organized by Other Russia, a broad umbrella group that includes radical leftists, like National Bolshevik Party leader Eduard Limonov, as well as former chess champion Garry Kasparov and former Prime Minister Mikhail Kasyanov.
Protesters Accuse Putin
They accuse the Kremlin, under President Vladimir Putin, of stifling civil liberties ahead of the 2008 presidential election.
Demonstrators dubbed their rally the March of Dissent.
Several hundred of them broke through a security cordon and marched along the city's main avenue before riot police armed with truncheons moved in, detaining scores of people.
RFE/RL Russian Service correspondent Tatyana Voltskaya, who covered the rally, said police actions to try to halt the event probably contributed to the violence.
“When I came I saw a very tight police cordon. The police had left only two very narrow corridors for people,” she said. “I would say that to be inside it was very unsafe because of a possible stampede."
“Police officers were speaking through megaphones," she said. "They urged people to leave the square and not to disturb the public order. It was not possible to hear what they were saying because the crowd was shouting ‘Disgrace, disgrace!’”
Protesters shouted "Russia without Putin!" and "Out with the corrupt authorities!" One group of protesters was dressed like clowns.
Police Beat, Detain Protesters
Voltskaya said violence broke out as thousands of people marched through the streets.
“The police were beating the protestors, pushing them away," she said. "They did not allow them to go as they planned to Suvorov Prospekt, so people turned to Nevsky Prospekt.”
Among those detained by police today was National Bolshevik leader Eduard Limonov.
Voltskaya says it was a disturbing experience to observe special police forces beating demonstrators, grabbing their flags and banners.
The demonstration comes ahead of local elections in St. Petersburg on March 11, elections to the national parliament this winter, and a presidential election on March 2 of next year in which Putin cannot take part, according to the current constitution.
(with material from agency reports)