By Maria Tsvetkova, Denis Pinchuk and Jack Stubbs
MOSCOW (Reuters) - Russian police wrestled opposition leader Alexei Navalny into a patrol wagon moments after he appeared at a rally on Sunday to urge voters to boycott what he said would be a rigged presidential election in March.
Navalny has little chance of influencing the election, likely to be won comfortably by President Vladimir Putin, but his ability to use social media to mobilize crowds of mostly young protesters in major cities has irked the Kremlin.
The numbers who showed up at protests across Russia on Sunday -- some shouting "Putin is a thief" -- were lower than previous demonstrations Navalny had staged, according to estimates from Reuters reporters, indicating the momentum may have shifted away from him.
Video footage posted on social media showed Navalny appear on Moscow's main thoroughfare, Tverskaya Street, a few hundred meters from the Kremlin, to join several hundred supporters taking part in the protest, which the authorities had said was illegal.
He had only walked a short distance when he was surrounded by police officers wearing helmets. They grabbed him and forced him to the ground on the pavement, and then dragged him feet first into the patrol wagon, the video footage showed.
Navalny's personal Twitter feed carried a post to his followers saying he had been detained. "That does not matter. Come to Tverskaya. You're not coming out for me, but for your future," the post said.
He was taken to a police precinct in central Moscow, Navalny's Internet site reported. Police said in a statement he would be charged with violating laws on holding demonstrations. The maximum penalty Navalny faces for the offence is 30 days in jail.
Navalny emerged as a threat to the Kremlin's tight grip on power on June 12 last year, when thousands of his followers defied police prohibitions to protest in cities across Russia. The scale of the protests, some of the biggest in six years, took the Kremlin by surprise.
On Sunday, around 1,500 protesters converged at Manezh Square, adjacent to the Kremlin, but were blocked from getting any further by metal barriers and dozens of police in riot gear. The crowd split up and headed off in several different directions, with the numbers thinning out as night fell.
Hundreds of people also protested in St. Petersburg, Russia's second-biggest city, in Yekaterinburg in the Ural mountains, and other major centers.
By 1700 Moscow time (1400 GMT), police had detained 243 protesters nationwide, according to OVD-Info, a non-governmental group that tracks the arrests.
But Navalny, who says he has faced a campaign of harassment from the authorities, has struggled to mobilize the same numbers in the two nationwide protests he has called since the breakthrough protest in June.
In Moscow on Sunday, the protesters who had accompanied Navalny until his detention were at times hard to distinguish from the flow of shoppers and sight-seers on Tverskaya Street.
A Reuters reporter heard two American tourists asking if Putin "is coming to this parade?" A few meters from where Navalny was detained, a tour guide was telling a group of clients about the history of a pre-revolutionary mansion.
Putin, 65, has been in power, either as president or prime minister, since 2000, longer than veteran Soviet leader Leonid Brezhnev and outstripped only by dictator Josef Stalin.
Many voters credit him with restoring stability after the turbulence following the collapse of the Soviet Union, and of rebuilding Russia's influence abroad through his foreign policy.
Putin has said the presidential election will be fair, and that Navalny, lacking real popular support, is irresponsibly trying to foment social anger which could tip Russia back into instability.
Putin's opponents though say he is maintaining his grip on power by eroding democracy.
In the Urals city of Yekaterinburg, just under 900 miles (1,450 km) east of Moscow, around 1,000 protesters gathered in freezing temperatures to back Navalny's boycott call.
"No election? No to elections," a placard being held by one young man read.
About 200 protesters gathered in Vladivostok, on Russia's Pacific coast, alleging that the election would be skewed in Putin's favor.
"We have not been given any choice," said local resident Vladimir Redko, who turned up to the protest carrying a small child in his arms.
"And as a citizen of the Russian Federation, I feel it is my civic duty to declare my position, to say I disagree."
Earlier on Sunday, police forced their way into Navalny's Moscow office using power tools, citing reports of a bomb threat, an online feed run by Navalny's supporters showed.
(Additional reporting by Katya Golubkova, Gabrielle Tetrault-Farber and Polina Ivanova in Moscow, Natalia Shurmina and Anastasia Teterevleva in Yekaterinburg, and Mikhail Antonov in St Petersburg; Writing by Andrew Osborn and Christian Lowe; Editing by Jane Merriman and Keith Weir)