Anti-globalisation activists have rioted for a third consecutive night in Hamburg, even after G20 leaders had already left the northern German city.
Police used water cannon trucks again early on Sunday against rioters attacking them with iron rods and pavement blocks.
They arrested 144 protesters and temporarily detained another 144 people.
More than 200 officers have been injured in the violence since Thursday. The number of injured protesters was not clear.
German Foreign Minister Sigmar Gabriel condemned the violence, saying: "Germany's reputation is severely affected internationally by the events in Hamburg."
Mr Gabriel told Bild am Sonntag newspaper that a Europe-wide investigative team should search for suspects.
The overwhelming majority of the tens of thousands who took to the streets protested peacefully against the G20 summit.