Police fired water cannons and pepper spray at protesters near where the Group of 20 leaders are meeting in Hamburg, Germany, on Thursday.
Thousands of people are taking part in protests against the G-20 summit, which begins Friday.
The demonstrators had gathered at a riverside plaza in Hamburg, before beginning to march through the city.
Television reports showed protesters throwing stones, firecrackers and bottles at lines of police, and streets blocked by law enforcement equipment. The officers were shown using water cannons to push protesters back.
Hamburg had reinforced its police force, and had 20,000 officers on hand to patrol the northern German city. Police said they expected more than 100,000 protesters for the summit, the Associated Press reported.