Hamburg was hit with its first clashes on Sunday evening ahead of what is expected to be a week of protests during the G20 summit.
A small riot erupted as police attempted to clear a protest camp in a city park, police said. The camp was approved as an assembly point but overnight camping had been forbidden.
In the evening, about 600 people had gathered and were erecting tents against police orders. Clashes erupted when police intervened, with officers using pepper spray.
One man was arrested, police said, and activists said several people were injured.