Russia's Vladimir Putin took the oath of office Monday to become president for a historic third term.

At an elaborate Kremlin ceremony attended by some 2,000 dignitaries, Mr. Putin placed his hand on a copy of the constitution and swore to "respect and protect the rights and freedoms of the people."

Police tightened security in the capital, Moscow, ahead of the inauguration.

Mr. Putin had been Russia's prime minister since relinquishing the presidency in 2008. He won the country's March 4 presidential election by a landslide. The opposition disputes the victory, calling it a stolen election.

World leaders acknowledged Mr. Putin's victory with reservations, and international observers say the election was skewed in his favor.

Russian riot police clashed with demonstrators Sunday as an estimated 20,000 people protested against Mr. Putin.

Helmeted officers beat protesters with batons and hauled away dozens after skirmishes broke out between the two sides near a bridge leading across the Moscow river to the Kremlin.