The rally was intended to celebrate the first anniversary of demonstrations that grew into the biggest protests against Putin since he rose to power 13 years ago, but police were out in force to keep order and helicopters buzzed overhead.

Despite the risk of arrest, protesters stood clapping. Some chanted "Free political prisoners" and "Down with the police state." One unfurled a banner saying "crooks and thieves" - the popular name used to describe the Russian leadership.

Leftist Sergei Udaltsov and anti-corruption blogger Alexei Navalny were detained on the central Lubyanka Square, where witnesses said about 2,000 people gathered, and protest leaders Ilya Yashin and Ksenia Sobchak were detained on their way.

"The bloody regime has got to Sobchak," Navalny said before he was hauled away, referring to the 31-year-old former socialite who has joined the opposition even though her late father was once St Petersburg mayor and a close ally of Putin.

Moscow city authorities refused to authorise the protest and police in helmets and flak jackets told people to leave as they gathered for the rally despite the freezing cold.

"I don't know how many people are here but I am proud of each and every one of those who came here. The main thing is that people are here, that they are expressing their view and showing that they exist," Navalny told reporters.

One protester, a translator who gave her name only as Anna, brought her prayer book with her.

"I'm praying for Russia. God made us free. No one can take that away from us, or punish, detain or torture us for our political views," she said.

OPPOSITION ACCUSES PUTIN OF CRACKDOWN

The protests began a year ago over suspicions that Putin's United Russia party benefited from widespread vote-rigging when it won a parliamentary election. Opponents say Putin's domination of Russia is dooming the country to economic and political stagnation at a time when it needs reform.

The protests reached their peak at the turn of the year but started to wane after Putin won nearly two-thirds of votes in the presidential election in March, enabling him to return for his third presidential term after four years as prime minister.

The demonstrations accelerated the birth of a civil society two decades after the collapse of the Soviet Union, but opposition leaders accuse Putin of clamping down on dissent and freedom of expression since he began his new presidential term in May.

Laws passed since May broaden the definition of treason, increase the punishment for protesters who step out of line, and tighten control over the Internet and on campaign and lobby groups that receive foreign funding.

Several opposition leaders, including Navalny, face criminal charges that they say are politically motivated and intended to intimidate them into giving up their opposition activities.

Putin, 60, denies any crackdown. He maintains a grip on state media and has retained support in the industrial and provincial areas that are his traditional power base.