As Chicago prepares for the NATO Summit this weekend, police in other cities, and even some veteran demonstrators are offering some advice: “beware of the black bloc”.

Black Bloc demonstrators are self-styled anarchists who appear at large demonstrations, often at world summits, and are known for inciting violence.

"This autonomous group of 50 youth, mostly boys, dressed in black, bent on being scary and provocative came and stole the entire narrative of the story," said John Sellers of Seattle’s Ruckus Society.



Sellers was the leader of a large demonstration against the World Trade Organization in 1999. He calls Black Bloc demonstrators "parasites" because of their tactics of blending with other demonstrators before lashing out on their own.

"They don’t want to take any responsibility for what they’re doing," Sellers said. "They need thousands of people who are willing to take responsibility for their protest to hide behind."

Worst of all, Sellers said, the Black Bloc protesters tip demonstrations into violence which would otherwise have remained peaceful.

"I think they create situations that are violent for the people around them, that didn’t go to get into a violent situation," he said.

Seattle knows the phenomenon all too well. In 1999, the WTO protests became virtual symbols for the anti-globalization movement, and those images were often violent.

"The Black Bloc are, as you point out, dedicated to violating every rule that you can imagine," said former Seattle Police Chief Norman Stamper. "They want to upset non-violent protesters."

Former Chicago Chief of Patrol James Maurer agrees. Maurer says one of the biggest difficulties with the Black Bloc is their tactic of striking with violence or vandalism, then attempting to blend into the crowd.

"They’ll intermix in a parade route and you’ll have everything under control, and everybody will be walking in their parade route, and then a group will break off someplace and start mayhem," Maurer said. "They’ll have these outfits on, and cause a little trouble, then they’ll throw the outfits off and be back in the crowd dressed normal, so you don’t know who they are any more."

Police said that while many self-styled Black Bloc protesters hold genuine anarchist beliefs, many appear at rallies only intent on causing mayhem. They consider these individuals wild cards who can steer an otherwise peaceful demonstration into chaos.