Anti-governments protests in Russia were hampered by a show of force by local police who detained more than 1,000 people in a crackdown.
It was a setback for government opponents seeking to channel public anger over the weekend's election, widely seen as slanted in favour of Prime Minister Vladimir Putin's ruling United Russia party, into a powerful protest movement.
Mr Putin pressed ahead with his attempt to return to the presidency next year, filing papers to register his candidacy.
A day after police dispersed protesters in central Moscow and seized others before they could even reach the rally, detaining more than 300, opposition activists had planned a new demonstration at the same site 24 hours later.
But hundreds of helmeted riot police blocked off the square after nightfall, pushing back reporters and shouting through loudspeakers: "Respected citizens, please do not stop, walk on your way so as not to hinder others."
In St Petersburg, about 250 people protested, most of them youngsters, shouting "Shame!" Police detained about 70.
Kremlin opponents are trying to maintain momentum after 5,000 people turned out on Monday night for the largest opposition protest in Moscow in years, demanding fair elections and chanting "Russia without Putin!"
Police and Mr Putin's spokesman have said unapproved protests will be stopped. The Interior Ministry said some 50,000 officers and 2,000 ministry troops remained in Moscow after the election.
Gorbachev urges re-run of elections
Former Soviet leader Mikhail Gorbachev has called for Russia's elections to be re-run due to fraud.
"The results do not reflect the will of the people," Gorbachev, president when the Soviet Union collapsed two decades ago, told the Interfax news agency.
"Therefore, I think they (Russia's leaders) can only take one decision - annul the results of the election and hold new ones."
US Secretary of State Hillary Clinton said the elections were neither free nor fair.
The Russian foreign ministry protested that Ms Clinton's comments were "unacceptable", while top politician Konstantin Kosachev said they were drawn from "one of the darkest pages in the recent history of Russia-US relations".
Internet-based protesters vowed further demonstrations in the days to come, despite a warning by police that participants in unsanctioned protests would be arrested.
A group "for honest elections" said on its Facebook page that a new demonstration would take place in central Moscow on Saturday afternoon.
More than 5,000 members of the Facebook group have already said they would attend.
Russia's liberal newspapers hailed the protests, saying the opposition had finally found a voice against the domination of Mr Putin.
"We prevented the real political process and built cardboard scenery instead. But it's dangerous to hold back a natural process: now it's payback time," wrote liberal daily Vedomosti in an editorial.
The Nezavisimaya Gazeta added: "The middle class has started to express its discontent, which is an extremely alarming signal for the authorities."
