Russian riot police clashed with demonstrators Sunday as an estimated 20,000 people protested on the eve of Vladimir Putin's inauguration for a third presidential term.

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.

Those detained include three key leaders in the protest movement against Mr. Putin — Alexei Navalny, Boris Nemtsov and Sergei Udaltsov. Officials say at least four officers were injured.

Mr. Putin has been Russia's prime minister since relinquishing the presidency in 2008. He won the country's March 4 presidential election by a landslide, but 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 clearly skewed in the former president's favor.