MOSCOW: Vladimir Putin took the oath of office for a third term as Russia's president on Monday, saying he considers ''service to the fatherland and our nation to be the meaning of my life''.

Putin 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.

Despite unprecedented security measures in the center of Moscow, several thousand opposition activists tried to protest along the route Putin's motorcade took to the Kremlin and were met by helmeted riot police. Dozens of demonstrators have been detained.

Putin's inauguration in a brief Kremlin ceremony 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 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 indicate 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 hand on a red copy of Russia's constitution, Putin stated his commitment to democracy.

''We want to live and we will live in a democratic country that will offer opportunities and freedom for everyone's life and labor,'' he said in a grand Kremlin hall before 3,000 invited guests.