Vladimir Putin has been sworn in as Russia's president for a third term after four years as prime minister.

Putin was first elected president in 2000 and the new six-year term will keep him in power until 2018.

Putin's inauguration on Monday comes a day after an opposition protest drew more than 20,000 people, fewer than the mass demonstrations that preceded his election but still a sign that the anger over Putin's return to the Kremlin has not faded.

The protest turned violent when some demonstrators tried to march toward the Kremlin and helmeted riot police beat back the crowds with batons and detained more than 400 people.