ST. PETERSBURG, Russia -  Riot police clashed with opposition supporters at the end of an anti-Kremlin protest in Russia's second-largest city Sunday, beating some and hauling them into police buses.

It was not immediately clear what sparked the violence after the rally, which authorities in St. Petersburg had authorized and took place under a heavy police presence. At least one helicopter hovered above the demonstrators. No information was immediately available on how many people were injured.

After one of the clashes, an elderly woman comforted a young man who lay on the sidewalk with blood on his face. "So this is what they call democracy,'' a passerby remarked.

Several of the rally's organizers were detained, at least one of them before the event started. The RIA-Novosti news agency cited police sources as saying more than 100 people had been detained.

Although city authorities gave permission for the rally in a square on the edge of central St. Petersburg, they had banned plans for the demonstrators to march afterwards to the city's government headquarters.

Police trucks and helmeted officers blocked the planned march route. At the end of the 90-minute rally, organizers did not call on them to march along the banned route, but suggested they go on their own to the city government building over the next few days. When the rally dispersed, most participants went to a nearby subway station, where clashes broke out.

In one, police chased a group that included Sergei Gulyayev, a former member of the city legislature who had been arrested at a protest in March.

Police grabbed some members of the group and hit them over the head with truncheons before putting them on buses. Gulyayev was among those taken away.

In another clash, police charged a group holding a banner professing love for the city.

Andrei Dmitriyev, the local head of the banned National Bolshevik Party, told The Associated Press he had been beaten and detained. Also detained was Maxim Reznik, a local head of the liberal Yabloko party.

The violence came a day after clashes at a similar opposition protest in Moscow, where police detained at least 170 people.

"Yesterday, it became clear that the authorities won't be making any concessions,'' Eduard Limonov, head of the National Bolshevik Party, told Sunday's rally. "They have started a war on people.'' Limonov's whereabouts after the rally were not immediately known.

The weekend protests were part of a series of "Dissenters' Marches'' called by the Other Russia umbrella group that brings together an array of opposition factions, including one led by former world chess champion Garry Kasparov.

Kasparov was among those arrested in Moscow and was released late Saturday night after being fined the equivalent of about US$40.