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Helmeted riot police round up hundreds of protesters, including some whose only apparent crime is wearing white ribbons of opposition. A teacher who spoke out about election rigging is dragged into court and fined. Now a new law signed by President Vladimir Putin on Friday raises fines for participating in unauthorized protests 150-fold, to nearly the average annual salary in Russia.

Putin has cracked down on the opposition since returning to the presidency, and he seems to be betting that by threatening demonstrators with prison time and harsh fines he can quash the street protests that have posed an unprecedented challenge to his 12-year rule.

Putin said that the law is designed to safeguard Russians from "radicalism."

"In guaranteeing citizens' right to express their opinion, including in street rallies, society must protect other citizens, the general public, from radicalism," he said in televised remarks. He added, however, that the law may be amended if necessary.

His strategy faces a major test on Tuesday when the opposition plans its first mass demonstration since he began his third presidential term on May 7.

Some opposition leaders contend that the tough line will help their cause by fueling anger and bringing more people out for next week's protest. Others say the repression will scare away the middle-class protesters who turned out in the tens of thousands for peaceful demonstrations this winter.

Putin, for his part, is refusing any talks with the opposition.

"He understands only one language, the language of force, and therefore he perceives any normal discussion and any rational compromise as personal weakness," said Yevgenia Chirikova, an environmental activist who has campaigned against Kremlin-backed road construction that is destroying a forest outside Moscow.

Chirikova and Ilya Yashin, who recently spent 15 days in jail for leading unsanctioned protests, were among a group of opposition leaders who met Thursday in Moscow to discuss the implications of the new law, which jacks up fines to 300,000 rubles ($9,000).

Yashin tried to ease worries, saying protest leaders would collect donations for those punished, as was done within hours when St. Petersburg teacher Tatyana Ivanova was fined 30,000 rubles ($1,000) last week. Ivanova was found guilty of damaging the reputation of an education department official she had accused of pressuring her and other poll workers to falsify the December parliamentary vote.

The anti-Putin protests broke out after the December election, which observers said was riddled with fraud in favor of Putin's party, and continued in the run-up to the March presidential vote. As many as 100,000 people turned out in the frigid cold for demonstrations demanding free elections, and the streets of Moscow rang with cries of "Russia Without Putin" and "Putin Is a Thief."

Although he was denied a majority in Moscow, Putin won the election to return to the Kremlin post he had held from 2000 to 2008 before moving into the prime minister's office to avoid violating a constitutional ban on more than two consecutive terms.

With the election over, the protest movement seemed to fade.

But on the eve of Putin's inauguration, an opposition march and rally drew tens of thousands, far more than either the organizers or the police had expected. The demonstration turned violent after police restricted access to the square where the rally was to be held. Bottles and pieces of asphalt were hurled at police, who struck back by beating protesters with truncheons and detaining more than 400. Some demonstrators were dragged away by their hair. Opposition leaders claim the clash was provoked by pro-Kremlin thugs.

In the days that followed, police chased opposition activists around the city, detaining hundreds.

Then the crackdown eased, as the authorities allowed the opposition to set up camp on a leafy boulevard. But there were strings attached: The organizers could not put up placards or make political demands, since that would technically turn the camp into an unsanctioned protest.

The authorities tolerated the camp for about a week before getting a court to rule that the activists were creating a mess in the neighborhood, giving police the legal right to disperse them.

The anti-protest legislation also provides police with new powers against such Occupy-style camps. "Large-scale public gatherings" can be banned and the organizers fined if they disrupt public order.

In a Levada poll released Thursday, 65 percent said they expected the protests to continue, although they differed on how likely the protests were to intensify or spread. The poll, conducted May 25-29 among 1,604 people across Russia, has a margin of error of 3.4 percentage points.

The bill was rushed through the Kremlin-controlled parliament this week in an effort to get it in place before Tuesday's big protest.

Some opposition leaders had held out hope that Putin would refuse to sign it. Others, however, had said they had no illusions, pointing to a comment by Putin's spokesman Dmitry Peskov after the violence on the eve of the inauguration. Protesters who hurt riot police, he said, "should have their livers smeared on the asphalt."

It was the kind of language that Putin likes.

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Irina Titova in St. Petersburg contributed to this report.