An online election to choose a "shadow parliament" opposed to Russia's President Vladimir Putin was disrupted on Saturday by a cyber attack, activists said.
"Today we already know that there are some problems with the server, there are some attacks," Sergei Udaltsov, a prominent protest leader, said at rally on Saturday.
Opponents of Putin say elections in Russia are rigged in favor of his ruling party and are instead holding their own internet contest which they hope will reinvigorate the flagging opposition movement.
Information on their website, www.cvk2012.org, told visitors there could be problems casting their votes and to try again later. Organizers and activists did not suggest who was behind the attacks.
The Kremlin has said it will ignore the opposition vote.
The rally by several hundred people in central Moscow was to support the internet elections and protest the detaining of opposition activists. Pussy Riot member Yekaterina Samutsevich, recently freed from jail, appeared, Interfax news agency said.
Russian investigators have began criminal proceedings against Udaltsov, saying a documentary on a pro-Kremlin TV channel had uncovered evidence he had plotted mass disorder. His aide Konstantin Lebedev has been charged with plotting mass disorder and violence. Both men deny the accusations.
"We are witnessing a severe attack on the opposition by the authorities ... criminal investigations are being launched," said Udaltsov.
Leonid Razvozzhayev, who was also named in the documentary, has been detained in Kiev, Interfax quoted Razvozzhayev's boss, lawmaker Ilya Ponomarev, as saying.