An anti-corruption crusader challenging Russian President Vladimir Putin was violently arrested Sunday at a demonstration in Moscow.

Police pounced on Alexei Navalny, considered the most serious challenger to strongman Putin, just moments after he appeared at a rally in the city’s Pushkin Square.

Video shows a half-dozen cops descend on Navalny and drag him into a police bus while using their batons to cross-check protesters who try to intervene.

“I have been detained. But that doesn’t matter,” Navalny’s Twitter read later in the day. “You are coming out not for me but for yourself and your future.”

The arrest came as thousands of Russians marched in more than 100 cities, demanding a boycott of the March 18 presidential election after Navalny was disqualified in December for a fraud conviction that supporters say is political retribution.

Demonstrators took to the streets in major cities such as Moscow and St. Petersburg, as well as remote regions including Yakutsk in Siberia, where sizable crowds braved temperatures of 49 below zero.

The Russian government deemed Sunday’s protest illegal, and Navalny is expected to be charged with a public-order violation punishable by 20 days in jail.

The Kremlin says it is investigating whether Navalny broke the law in calling for the boycott.

Earlier Sunday, police sawed through a door to enter Navalny’s Fund to Fight Corruption headquarters — where a livestream of protests was being produced — claiming they had received a bomb threat. They arrested two workers there.

Putin has been president more than 18 years and is expected to handily win another term in March. His approval rating is 80 percent, according to state-run polls.

With Post wires