As Nemtsov was seized by five police officers, the crowd chanted "Russia without Putin".
Nemtsov was reportedly released a few hours later.Protesters outnumbered
Only about 500 activists were able to take part in protest marches in Russia's second city and were vastly outnumbered by riot police.
|"We are ruled by the United Russia gang. They have taken away the elections" |
Lyubov Chilipenko, St Petersburg protester
The city authorities had not given permission for the march to go ahead and streets in the centre of the city were blocked by riot police and snow clearing trucks.
"What we are seeing now in St Petersburg resembles a military operation," Olga Kurnosova, a co-ordinator of the Other Russia coalition of opposition groups, said. "It shows the authorities' fear."
The "march of the discontented" brought together Putin's opponents into one movement, including Other Russia, free-market parties such as SPS and Yabloko, as well as anarchists and radical socialists.
"They have started a war with the people," Tamara, a 72-year-old who took part in the march, said.
"Putin is very bad - look at the poverty in the country. Pensioners are forced to rummage in the dustbins."Kasparov jailed
On Saturday, Garry Kasparov, leader of Other Russia and former world chess champion, was arrested when police broke up a march of about 3,000 in Moscow. Activists said about 60 people were detained at that march.
Kasparov has been sentenced to five days in jail.
Putin has vowed to step down as president next year after his second consecutive four-year term in office but has said he will use the United Russia party to preserve influence after he steps down.
He will head the electoral list for the party in December's parliamentary election.
"We are ruled by the United Russia gang. They have taken away the elections," Lyubov Chilipenko said at the St Petersburg march.
The demonstration came ahead of a visit by Putin to the city on Monday.