Vladimir Putin filed candidacy papers for a March 4 presidential election on Wednesday while his opponents prepared for more protests over a parliamentary vote they say was rigged in favour of his ruling party.
Some 5,000 people turned out on Monday night for the largest opposition protest in Moscow in years, many chanting "Russia without Putin!". More than 300 were detained on Tuesday after a similar demonstration.
Putin submitted his registration documents in a brief and nearly silent visit to the Central Election Commission headquarters in a lane near the Kremlin.
Registration as a candidate is a formal step towards what could be 12 more years in the top job for Putin. He was president from 2000 to 2008 and is now prime minister, but the 59-year-old remains Russia's paramount leader.
But voters bruised Putin on Sunday by sharply reducing his party's majority in the State Duma lower house, which he uses as both an instrument of power and a source of support.
United Russia received 49.4 percent of the votes, according to the election commission, down from 64.3 percent in 2007. It will have 238 seats in the 450-member Duma, down from 315 now.
Protests over the election, fanned by fraud accusations that have spread on the Internet, have underscored anger at United Russia and unhappiness among some Russians at the prospect of Putin's almost certain return to the presidency.
Police swamped the centre of the capital on Tuesday and said they detained more than 300 people on and around Triumph Square, where hundreds or even thousands more tried to protest. Another 250 were detained in St Petersburg.
Two protest leaders sentenced to 15 days in jail for their roles in Monday's rally appealed those sentences on Tuesday. Ilya Yashin's appeal was rejected and a decision was expected later on the appeal of Alexei Navalny, also a prominent anti-corruption blogger.
Opposition groups said they were planning another protest in the same place on Wednesday evening despite earlier warnings from police and Putin's spokesman that unsanctioned protests would be stopped.
Reinforcements of 50,000 police and 2,000 troops were stationed in the capital ahead of the election and there are no plans for them to leave for the time being, a Moscow city police spokesman said on Wednesday.
Buses of police were parked near the square on Wednesday.
Monday's rally drew many people who had never demonstrated before, according to participants. Kremlin opponents hope protests will gain momentum by bringing more Russians who are unhappy with Putin and his party on to the streets.
Putin tacitly acknowledged many Russians' desire for change on Tuesday by promising to reshuffle the government after the presidential election. But the Kremlin had already signalled that would occur and Putin promised no immediate action.
Putin and President Dmitry Medvedev, the protege he tapped as successor when term limits barred him from seeking re-election in 2008, revealed in September that they plan to swap jobs next year, with Medvedev taking over as prime minister.
Putin remains Russia's most popular politician and is likely to win a six-year presidential term, after which he could run again, potentially serving until 2024.
European observers said the election was slanted in favour of United Russia and marked by indications of ballot-box stuffing.
U.S. Secretary of State Hillary Clinton, in Lithuania for an Organisation for Security and Cooperation in Europe (OSCE) meeting, suggested on Tuesday that it was neither free nor fair.
Russia Foreign Minister Sergei Lavrov said on Wednesday that Clinton's actions showed "disrespect" to the 56-member OSCE.
"This (OSCE) is not Hyde Park, this is not Triumph Square in Moscow, where speakers arrive to pour out their soul and then turn around and leave, not listening to others," he said.
He was referring to London's Hyde Park and its famed "Speakers' Corner" and to the square in Moscow that was the site of Tuesday's protest.
(Additional reporting by Thomas Grove, writing by Steve Gutterman, editing by Robert Woodward)