At least 33 people, including the leader of Russia's opposition Yabloko party, have been detained by police at an unsanctioned rally in the center of Moscow to mark the Constitution Day holiday, Radio Free Europe reported on Saturday.

Sergei Mitrokhin was detained at the December 12 gathering on Moscow's Pushkin Square as civil-society activists tried to carry out a March of Change demonstration despite a refusal by city authorities to allow the action.

Several other Yabloko activists were also detained, as were prominent rights activists Lev Ponomaryov and Georgy Satarov.

Constitution Day marks the anniversary of a 1993 referendum under which the current Russian Constitution was adopted. It was a federal holiday until 2004 when it was downgraded to a "day of note."

Sources: MENA