Masked protestors smashed the windows of a police car and missiles were thrown at police in the first signs of violence during an anti-globalization rally on Saturday in the northern German port of Rostock to protest at this week's G8 summit.
Police reported that around a dozen people, their faces covered to prevent identification, had attacked a police car near the harbour of the town, where the marchers were to hold a rally later in the day.
Bottles and stones were also thrown at the 5,000 uniformed officers monitoring the marchers, heading along two routes to the harbour, police spokesman Axel Falkenberg said.
Windows were broken at a bank along the route.
Earlier police said that fewer than 30,000 anti-globalization protestors were taking part, well down from the 100,000 expected by the organizers.
Many of the demonstrators, who include peace activists and environmentalists, arrived at two main gathering points on special trains or by bus, while others had camped near the city overnight.
The protest, under the slogan, "Another world is possible," was being held to oppose the G8 summit being held from Wednesday to Friday in the resort of Heiligendamm some 25 km away.
In the city itself, shopkeepers and traders had battened down the hatches, protecting display windows with wooden boards.
Heiligendamm has been completely cordoned off, provoking concerns that democratic rights to demonstrate were being infringed.