St. Petersburg, Russia (CNN) Russian police arrested 290 protesters in 26 cities as marches took place against President Vladimir Putin on the leader's 65th birthday, according to independent monitoring group OVD.

Thousands attended the marches on Saturday, held by supporters of Putin's only real political opponent, activist Alexei Navalny, who is serving a 20-day jail term for organizing an unauthorized public meeting.

It is very difficult to legally organize protests in Russia, where public assembly laws have been tightened several times since Putin's current presidential term began in 2012.

Demonstrators gather by a statue of poet Alexander Pushkin in an anti-Putin rally in Moscow on Saturday.

Navalny, 41, has said he intends to run for president in the next election, despite carrying an embezzlement conviction that prevents him from doing so. He claims that conviction was bogus and politically motivated to block his presidential bid.

Russian activist Alexei Navalny in court for organizing unauthorized rallies, in Moscow on June 12, 2017.

Navalny has been jailed three times this year.

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