Protesters have wrecked a bank and media satellite trucks after breaking away from a demonstration by anti-globalists outside an EU summit in Belgium.
Meanwhile, 60 German protesters, headed for the Brussels demonstrations, have been detained in Liege, just outside the German border.
Tens of thousands of protesters, representing various environmental, trade and other protest groups, have marched through Brussels.
The demonstration has been kept away from the royal palace in the suburb of Laeken, the venue of the meeting by the 15 EU leaders.
Several protesters, dressed in black and wearing balaclavas, broke away from the demonstration and smashed windows at a nearby bank. Two media satellite trucks were also damaged police said but added that no arrests were made.
Under the banner of "another Europe for another world," activists, including anti-globalists grouped in an organization calling itself D14, demonstrated. They said EU policies go "against the aspirations of the people" because they pursue capitalism, free trade and the American economic model.
Protest organizer Olivier Bailly said the march protesting everything from capitalism to peace efforts in Central Africa.
"Through these different struggles we want to give a clear message, a more social and human Europe," he said.
Police, wary of a repeat of violent summit protests at previous EU gatherings, took a heavy-handed approach to security and deployed 3,000 officers.
A large area of Brussels around the Laeken Palace was blocked off to traffic. Police helicopters and water cannons were deployed alongside riot police and horse patrols.
Protest organizers fielded their own security stewards to help spot and defuse any potential violence. Their marching route was limited to only 5 kilometres (3 miles), a length that Bailly called unfairly short.
With threats of possible terrorist attacks still looming large, the Belgian air force was also placed under alert as EU leaders started their meetings.