Tens of thousands of demonstrators around Russia are calling for a re-run of last weekend's parliamentary elections which were officially won by the governing Russia United Party.
The biggest protest is in the capital, Moscow, where up to 50,000 people are reported to have gathered.
Many waved flags and carried white carnations.
The Moscow demonstration is being described as the biggest anti-government rally there since the fall of the Soviet Union.
Security is tight and riot police have been out in force.
Hundreds of security trucks blocked off central squares while helicopters patrolled the skies.
Thousands more demonstrators turned out in freezing temperatures in other cities across Russia in a rare show of defiance against Prime Minister Vladimir Putin.
A BBC correspondent says Mr Putin has never experienced popular protests like this before.
Demonstrations immediately after last weekend's elections led to more than 1000 arrests.
The protesters claim there was widespread election fraud and vote rigging.
The United Russia Party won 49% of the vote and the Communist Party was a distant second with 19%.