Tens of thousands of Russians flooded downtown Moscow on Saturday to demand an end to Prime Minister Vladimir Putin's rule, casting a strong challenge to his bid to reclaim the presidency in March.

Protesters wearing white ribbons and holding placards reading "Russia Without Putin!" and "For Free Elections" marched to a square across the river from the Kremlin where a rally was held.

Saturday's crowd appeared to be even bigger than two similar rallies held in December, despite temperatures plunging to minus 4 Fahrenheit (minus 20 degrees Celsius).

The previous rallies _ the second of which drew 120,000 people, according to organizers _ were the biggest in Russia since the protests 20 years ago that paved the way to the collapse of the Soviet Union.

The authorities have sanctioned Saturday's march, even though they had rejected the organizers' earlier request to gather just outside the Kremlin.

"So many of us have come that they can's arrest all of us," said 56-year old protester Alexander Zelensky.

He and his wife, Alyona Karimova, said they had begun preparations to emigrate to Canada in the fall, but then changed their minds and decided to stay in the hope that the nation will eventually move toward democracy.

"This is going to be a gradual process, but we believe it will eventually lead to democracy and free elections," Karimova said.

Protesters, many bundled in fur coats against the cold, chanted "Putin, go away!" and "Russia without Putin!" Communists and nationalists also joined the protest, waving big flags.

The protests in December were triggered by evidence of fraud in favor of Putin's party in December's parliamentary election. Putin has ignored the demands for a repeat election, but he has sought to assuage the mostly urban middle-class protesters' anger by making vague promises of liberalization.

Putin also has sought to consolidate his core support group of blue-collar workers, farmers, public servants and the elderly with frequent meetings with pre-selected groups of people, which received lavish prime-time coverage on state-controlled nationwide television stations.

The race is pitting Putin against three leaders of parliamentary parties, who had run against him in the past, and one fresh face _ the billionaire owner of New Jersey Nets basketball team, Mikhail Prokhorov. Prokhorov joined Saturday's protest against Putin, but refrained from speaking at the rally.

None of the contenders is expected to pose any serious challenge to Putin, whose ratings are now hovering just below 50 percent needed for a first-round victory in the March 4 election. If Putin fails to win an outright victory, he will face a runoff three weeks later, most likely with Communist Party chief Gennady Zyuganov, a rival he will easily defeat.

But protesters at Saturday's rally denounced the race as illegitimate, saying that tight controls over the political scene imposed by Putin during his 12-year rule have removed any genuine political competition.

"These elections are false and illegitimate," said one of the opposition leaders, Ilya Yashin.

Grigory Yavlinsky, leader of the opposition Yabloko party who was barred from the race by election authorities, said the fight will not end after the presidential election.

"We are defending the future of our country," he said. "Our foes will soon see that it's only the beginning."

In an apparent attempt to demonstrate a massive public support for Putin, his backers gathered across town, but their rally only drew about 15,000. Municipal workers, union activists and teachers who showed up there said they came of their own will, but some admitted they had been asked by authorities to attend.

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Nataliya Vasilyeva contributed to this report.