Prominent Russian activist Alexei Navalny is promising a campaign of civil disobedience against what he says was a stolen presidential election by Vladimir Putin.

Navalny says tens of thousands of people will turn out for street protests in Russian cities and will keep up the protests until their demands are met.

Police arrested Navalny and hundreds of other opposition supporters at an anti-Putin rally in Moscow Monday. He was later released. Many of the protesters chanted “Russia without Putin” and “Power to millions, not to the police.”

Vladimir Putin's spokesman, Dmitry Peskov, defended the arrests saying police acted within accordance of the law. He said the opposition actions consisted of two parts — legal and illegal — adding that in both cases police acted legitimately and effectively.

Mr. Putin, who has been prime minister since relinquishing the presidency in 2008, won Sunday's election by a landslide. European observers say the election was clearly skewed in Mr. Putin's favor. They also report voting irregularities at one-third of the polling stations.

Mr. Putin calls the election an “open and honest struggle.”

World leaders acknowledge Mr. Putin's victory with reservations. European Union foreign affairs chief Catherine Ashton urged Russian leaders to address the “shortcomings” identified by international observers.

The U.S. State Department congratulated the Russian people on the completion of the presidential poll and said it looks forward to working with the president-elect. But it also urged Moscow to launch an “independent and credible” investigation of all reported electoral violations.