Disturbances broke out in Hamburg on Monday after a march protesting next week's G-8 summit and a meeting of EU and Asian foreign ministers, police said.
Officers used water cannon and batons to quell the unrest, and arrested 21 people, police spokesman Ralf Meyer said. Thirty more were temporarily detained, he said.
The march by some 4,000 anti-globalization demonstrators, according to police estimates, went relatively peacefully, although some protesters threw flares and bags of paint at police.
After the march ended, several hundred people streamed onto the city's Reeperbahn Boulevard. Cars were damaged by flying beer bottles and garbage containers were thrown into the street.