Alexander Zemlianichenko / Pool via AP

Russian President Vladimir Putin and former President Dmitry Medvedev, right, stand as an honor guard march during an inauguration ceremony at the Cathedral Square in the Kremlin in Moscow, Russia, on Monday.

Updated at 5:45 a.m. ET: MOSCOW --  Vladimir Putin was sworn in as Russia's president at a glittering ceremony on Monday, hours after clashes between police and thousands of protesters in the country's capital laid bare the deep divisions over his return to the Kremlin for six more years. 

In the latest demonstrations on Sunday, police detained more than 400 people, including three opposition leaders, after tensions boiled over at a rally attended by about 20,000 people across the Moscow river from the Kremlin. 

Police hit protesters on the head with batons as they tried to stop demonstrators advancing towards them, carrying metal crowd barriers and throwing objects. The crowd fought back with flagpoles before the police eventually restored order.

"Putin has shown his true face, how he 'loves' his people -- with police force," said Dmitry Gorbunov, 35, a computer analyst who took part in the protest. 

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Months of protests have polarized Russia and left Putin facing a battle to reassert himself or risk being sidelined by the powerful business and political elites whose backing is vital. 

Lavish reception
Putin, a former KGB spy, took his oath before nearly 2,000 guests in the Kremlin's St Andrew Hall, the former throne room with sparkling chandeliers, gilded pillars and high Gothic vaults, before being blessed by the head of the Russian Orthodox Church and taking charge of the nuclear suitcase. 

"We will achieve our goals if we are a single, united people, if we hold our fatherland dear, strengthen Russian democracy, constitutional rights and freedoms," Putin said in a five-minute speech after taking the oath. "I will do all I can to justify the faith of millions of our citizens. I consider it to be the meaning of my whole life and my obligation to serve my fatherland and our people." 

He was also due to host a lavish reception featuring only Russian food and drink. 

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Although he has remained Russia's supreme leader for the past four years as prime minister, Putin will take back the formal reins of power he ceded to his ally Dmitry Medvedev in 2008 after eight years as president. 

On Sunday, several thousand people staged a rally supporting Putin, seen by his backers as the only leader capable of defending Russia's interests on the world stage and the guardian of the economy at home. 

While Putin's critics have tired of a political system that concentrates power in one man, many of his supporters welcome his domination of the country of more than 140 million. 

Andrey Smirnov / AFP -Getty Images

Russian Police officers detain opposition supporters during a rally in Moscow on May 6, 2012. Russian riot police violently clashed with protesters at a rally on the eve of strongman Vladimir Putin's return for a third Kremlin term, arresting over 250 people including opposition leaders.

"Democracy is the power of the majority. Russia is everything, the rest is nothing!" Alexander Dugin, a Kremlin-aligned nationalist, told the pro-Putin crowd. 

Rifts
The rival rallies Sunday underlined the rifts opened by Putin's return to the Kremlin and protests that were sparked by allegations of electoral fraud but fuelled by many Russians' frustration that one man continues to dominate the country. 

The BBC reported that prominent opposition figures Alexei Navalny, Sergei Udaltsov and Boris Nemtsov were among the people detained Sunday. 

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Although the protests had lost momentum before Sunday's rally, they have given birth to a civil society, two decades after the collapse of the Soviet Union, that is gradually chipping away at Putin's authority. 

Putin, who will be 60 in October, grew up in Soviet days and worked as a spy in communist East Germany, is under pressure to show he can adapt to the new political landscape. Few think he has changed much -- if at all. 

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Putin has eased up on the choreographed macho antics that burnished his image at his peak in Russia, such as riding horseback bare-chested and shooting a tiger with a tranquiliser gun. 

Harder to shake off will be his habit of seeking total control and learning to cope with political opponents and a middle class demanding more political freedom. 

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He has to quell rivalries between liberals and conservatives battling for positions in the new cabinet under Medvedev, who is swapping jobs with Putin. The outcome of the struggle could help determine how far reforms go to improve the investment climate. 

Natalia Kolesnikova / AFP/Getty Images

Russians march along a street during an opposition protest rally in Moscow on Sunday. Russian riot police violently clashed with protesters on the eve of Vladimir Putin's return for a third Kremlin term.

The $1.9 trillion economy is in better shape than in most European countries but is vulnerable to any change in the price of oil, Russia's main export commodity. The budget is under pressure from Putin's lavish election spending promises. 

Putin has said he wants to attract more foreign investment by improving the business climate, reduce corruption and red tape, and end Russia's heavy dependence on energy exports. He has not spelled out how he will do this. 

Putin is likely, as in the past, to use tough anti-Western rhetoric on foreign policy to drum up support if times get tough in Russia. But he has never yielded his strong influence over foreign policy as premier, so a major policy shift is unlikely.

Reuters and msnbc.com staff contributed to this report.

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