Opponents of Vladimir Putin, the Russian president, have arrived in St Petersburg for a rally, a day after police broke up a protest in Moscow and arrested Garry Kasparov, an opposition leader.
Hundreds of riot police have been deployed in St Petersburg ahead of a rally by The Other Russia coalition, which accuses Putin of authoritarianism.
The authorities say the protesters are allowed to demonstrate only in a fixed location and that any attempt to march will be blocked.
Marina Litvinovich, an aide to Kasparov, said she and eight other members of The Other Russia were detained when they arrived in the city early on Sunday.
"We were arrested when we got off the train. They told us this was connected to the demonstration," she told Echo of Moscow radio by telephone.
"Formally, this was for a documents check, but then they told us we were on the federal wanted list... They did not say why."
On Saturday in Moscow, 9,000 riot police and soldiers were deployed to prevent less than 2,000 Other Russia activists marching to a central square.
Police arrested some 200 protesters including Kasparov, one of Other Russia's leaders and the former world chess champion.
He was released after being fined the equivalent of $40.
"It is no longer a country ... where the government tries to pretend it is playing by the letter and spirit of the law," Kasparov said outside the court building.
"We now stand somewhere between Belarus and Zimbabwe," he said.
He said he did not plan to go to St Petersburg.
Tensions are rising ahead of the March 2008 presidential election to replace Putin, who is constitutionally required to step down at the end of his second term.
Putin, who has overseen rapid economic growth in Russia, is popular and analysts believe that whoever the former KGB officer endorses as his successor will face little opposition.
Opponents say that overwhelming dominance is the result of a powerful state media machine, an economic bonanza from high world oil prices, and the growing power of the security forces.