The reformist wing of the Popular Front of Azerbaijan (AXCP) in turn says 43 of its supporters were seriously injured. Among them is the wife of AXCP Deputy Chairman Nariman Qasimzade.
The U.S. Embassy in Baku has condemned the police action, calling it "unjustified and unprovoked."
Thousands of opposition supporters had gathered on Baku's Qalaba (Victory) Square to protest the outcome of the 6 November legislative polls.
Official results show the opposition, which claims the vote was fraudulent, won 11 seats in the 125-member parliament. The ruling Yeni Azerbaycan (New Azerbaijan) party won 58 seats.