MOSCOW >> Vladimir Putin took the oath of office in a brief but regal Kremlin ceremony on Monday, while on the streets outside thousands of helmeted riot police prevented hundreds of demonstrators from protesting his return to the presidency.

Putin, 59, has ruled Russia since 2000, first as president and then during the past four years as prime minister. The new, now six-year term will keep him in power until 2018, with the option of running for a fourth term.

"I consider service to the fatherland and our nation to be the meaning of my life," Putin said in addressing 3,000 guests in a glittering Kremlin hall.

Despite unprecedented security measures in the center of Moscow, where streets were closed to traffic and passengers prevented from exiting subway stations, at least 1,000 opposition activists tried to protest along the route Putin’s motorcade took to the Kremlin. Many wore the white ribbons that are the symbol of the anti-Putin protest movement.

The demonstrators, separated into several groups, were met by helmeted riot police. A total of 120 were detained, including opposition leader Boris Nemtsov.

Putin’s inauguration came a day after an opposition protest drew more than 20,000 people, fewer than the mass demonstrations in the months that preceded his March election but still a sign that the anger over Putin’s heavy-handed return to the Kremlin has not faded.

Sunday’s protest turned violent when some demonstrators tried to march toward the Kremlin and riot police beat back the crowds with batons and detained more than 400 people. The use of force after the winter’s peaceful rallies indicates that Putin may take a harder line toward the protesters now that he is once again president.

After taking the oath of office with his right hand on a red copy of Russia’s constitution, which had been carried into the hall by goose-stepping Kremlin guards, Putin stated his commitment to democracy.

"We want to live and we will live in a democratic country where everyone has the freedom and opportunity to apply their talent and labor, their energy. We want to live and we will live in a successful Russia, which is respected in the world as a reliable, open, honest and predictable partner."

During his time in office, Putin has overseen dramatic economic growth and restored a sense of national pride after the instability and humiliations that followed the 1991 collapse of the Soviet Union. He also has retreated from the democratic achievements of the 1990s and imposed a political system that has stifled dissent.

Communist Party leader Gennady Zyuganov, who leads the largest opposition faction in parliament, warned that the government is radicalizing the protesters by refusing to take them seriously.

"The government must understand that the split in society is getting wider, and the anger over unfair elections and the lack of normal dialogue is growing. In this situation, radicalism is inevitable," Zyuganov said. "Any attempts to shut people’s mouths with the help of a police baton are senseless and extremely dangerous."

Putin has dismissed the Moscow protesters as ungrateful, pampered urbanites and agents of the West.

Dmitry Medvedev, who served as Russia’s president for the past four years as Putin’s junior partner, wrapped up his term with a short speech at the inauguration ceremony.

"I worked as I promised in taking the oath of office: openly and honestly in the interests of the people, doing everything I could so that they would be free and would look toward the future with confidence," Medvedev said.

Putin, as promised, began his new presidential term by formally nominating Medvedev as his prime minister. The parliament, where the Kremlin party holds a majority, was to vote on his nomination on Tuesday.

Putin’s wife, Lyudmila, who has rarely been seen in public in recent years, attended the inauguration ceremony. She was seated between Medvedev’s wife and the widow of Boris Yeltsin, who chose Putin as his successor in 1999.

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Associated Press writer Nataliya Vasilyeva contributed to this report.