MOSCOW (Reuters) - Protests against Vladimir Putin after his expected re-election as president this month could descend into street clashes if police crack down too hard on demonstrators, a senior opposition politician said.

FILE PHOTO: Supporters of Russian opposition leader Alexei Navalny attend a rally near the Kremlin in Moscow, Russia October 7, 2017. REUTERS/Andrey Volkov/File Photo

Polls show Putin, who has dominated Russia’s political landscape for 18 years, should win comfortably on March 18.

None of the seven candidates registered to run pose a threat. But Alexei Navalny, the opposition leader with the highest profile in the West, has been barred from standing over a suspended prison sentence for what he says is a trumped-up fraud charge, and is planning post-election protests.

“It’s realistic to expect violence,” Vladimir Milov, a Navalny ally and a former deputy energy minister, told Reuters in an interview.

“If police use excessive force, a significant number of people will fight back. We’re talking about clashes in Moscow and St. Petersburg. Several thousand people participating in clashes might be a realistic estimate.”

The demonstrators plan to demand a re-run with Navalny’s participation and to protest against what he and his allies believe will be widespread electoral fraud.

Polls show Navalny would not come close to unseating Putin, but his supporters believe he would pick up a sizeable protest vote and displace the Communists as the country’s second biggest political force behind the pro-Putin United Russia Party.

In the run-up to the election, Navalny, a 41-year-old lawyer, has organized the biggest anti-Kremlin protests in years. Police have detained hundreds at rallies that have drawn thousands, but large-scale violence has been averted.

Milov, who advises Navalny, said the opposition leader had been determined to ensure that the protests remained peaceful, but said he had overheard repeated discussion among demonstrators about responding to violence with violence.

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Some riot police, who had been bussed in to Moscow from distant provincial towns, had badly beaten protesters while detaining them during previous demonstrations, Milov said.

“At some point they (the police) will meet resistance,” he said. “People will maybe tolerate this one or two more times and then there will be clashes.”

Tens of thousands protested after Putin’s last election win in 2012. At one major Moscow demonstration that May, police detained more than 400 people after clashes that saw stiff jail sentences on charges of mass disorder handed down to some.

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The Kremlin says the police act proportionately to uphold public protest laws which stipulate that the location and timing of demonstrations must be agreed in advance.

Authorities in Moscow and St Petersburg have regularly rejected opposition applications for protests in the past, suggesting out-of-town venues on public safety grounds. The opposition has sometimes spurned those alternatives and gone ahead anyway.

Navalny, who has been repeatedly detained, was briefly taken into custody last month and said police had opened a case against him for organizing illegal protests. If found guilty, he could be jailed for 30 days.

Milov said protests were just one part of the strategy to try to bring political reforms to Russia and that, though large, they were for now not enough on their own to change the situation.

The authorities had so far exercised a measure of restraint when it came to breaking up protests, he said, because they were aware that a harsh crackdown could backfire.

“They (the authorities) are really behaving like they want to avoid mass violence,” said Milov, who said the opposition wanted to avoid violence too, even though they might gain support as a result.

“But who knows what will happen when the situation heats up. If they respond (to protests) heavily they would win using force, but there would be a reaction. It could act as a trigger event when a lot of people who were previously loyal shifted away from Putin because they did not feel secure anymore.”