MOSCOW (AFP) - The Russian protest movement on Monday vowed new demonstrations this week after a leading anti-Kremlin activist was given a fresh jail sentence following the latest mass rally against Vladimir Putin.

Tens of thousands of people took to the streets on Saturday in the second mass protest in a month, increasing the pressure on Putin for reform as he faces the biggest protest wave in Russia since the 1990s. "The Reckoning", the opposition New Times weekly wrote on its front cover, featuring a picture depicting Putin dressed as a Christmas gnome alongside leaders ousted in the Arab Spring like former Egyptian president Hosni Mubarak. The anti-Putin movement is not expected to hold new mass rallies until after the New Year. But the fresh prison term handed to Sergei Udaltsov, who leads the extreme-left wing Left Front Movement, has sparked huge anger within its ranks. Udaltsov had already served 15 days for taking part in a protest in Moscow against December 4 parliamentary elections won by Putin's party, a poll the opposition says were rigged.

But a Moscow court late Sunday ordered him to serve 10 more days in jail to finish a sentence he did not fully complete after being arrested at a demonstration in October, his lawyer Nikolai Polozov said. "Udaltsov got another 10 days. Shame!" Polozov wrote on Twitter late Sunday.

The radical activist had gone on hunger strike last week to protest his detention and had vowed to stage a fresh hunger strike if he was given a further term in jail, amid growing fears over his health. His supporters vowed on a Facebook page to hold a picket outside the Moscow court that ordered the sentence at 1300 GMT Monday in a bid to earn his release, although the page warned that the place and timing could be changed.

"We should be peaceful but determined. There should be a lot of us," said the post on the Facebook page Udaltsov.26.12. Another page called a potentially larger demonstration to support Udaltsov on Thursday evening in Moscow and Saint Petersburg: more than 1,000 people have already vowed to attend. While Udaltsov's left-wing views are not shared by all the opposition, protest movement leaders including blogger Alexei Navalny have shown a united front in expressing solidarity with his plight. Navalny has even promised that one million people will attend the next anti-Putin rally.

Hundreds of people were arrested in the first wave of protests after the elections but two mass rallies have since taken place with no unrest, smashing a taboo in Putin's Russia against mass opposition protests.

Putin's spokesman on Sunday told AFP that he respected the position of protestors but insisted the Russian prime minister still had the support of a majority of the people heading into presidential elections.

Putin plans to stand for a third Kremlin term in March 2012 polls after his four-year stint as prime minister, with President Dmitry Medvedev taking his current post in a job swap that has angered many Russians.

Medvedev's economic advisor Arkady Dvorkovich acknowledged in an interview with Moscow Echo radio that the protests would continue until the authorities reacted to their demands.