Police fired tear gas after some demonstrators threw stones and bottles and attacked police officers with sticks.
Police said at least 18 police officers were injured in the violent clashes, some of them seriously. There were no immediate details on detentions.
According to police, around 20,000 people gathered for the protest, organizers said it was 50,000.
In London, thousands of protesters rallied in the central part of the city today to urge G8 leaders meeting next week in Germany to keep their past promises about fighting world poverty.
Protesters staged "three minutes of noise" using whistles, alarm clocks, and mobile phones as they lined the banks of the Thames River and the bridges by the Houses of Parliament.
Organizers said up to 10,000 people took part.
The G8 summit is due to open in the nearby resort of Heiligendamm on June 6.
(Reuters, AP, AFP)