The 34-year-old leftist has been detained at least a dozen times in the past year after leading anti-Kremlin street protests, but he only recently has seemed to earn the attention and respect of Russia's main opposition leaders.

His jailing fed the anger during mass demonstrations in Moscow in December, and his release could help the opposition build momentum in the month remaining before the next big protest scheduled for Feb. 4.

Udaltsov urged his supporters to keep up the fight against Prime Minister Vladimir Putin's hold on power and to work for a democratic Russia.

"I'm calling on all of you not to ease up," Udaltsov said. "We will prepare new actions of civic peaceful protest. Without question we will go out on Feb. 4 and there will be other events."
He was arrested on Dec. 4, the day Russia held a parliamentary election during which observers said the vote was manipulated to allow Putin's party to retain its majority. Udaltsov was then kept in jail on charges related to a protest he led in October against the exclusion of opposition parties from the ballot. He spent much of his time in custody in a hospital after a hunger strike threatened his health.

During Putin's 12-year tenure as president and prime minister, opposition leaders have been marginalized and their protests routinely broken up by police. They are now capitalizing on public outrage over the election and a general weariness with Putin and the corruption his leadership has fostered.

The protests are posing the first serious challenge to Putin, who intends to return to the presidency in an election in March.

Udaltsov's supporters sent him off with shouts of "Russia without Putin," a popular chant at opposition rallies.

"It's a good slogan, but I would like to clarify that the problem is not just Putin," Udaltsov said. "It's the whole system that is rotten."

Udaltsov, a lawyer, has been a political activist since the late 1990s when he was allied with hardline communists. His great-grandfather was a prominent Bolshevik.