They go by the name Black Bloc, but even the masked, black-clad protesters who trashed store windows during an anti-Olympic march on the weekend wouldn't call themselves an organization.

The self-appointed "ninjas" of the anarchist movement have been a factor in European radical politics for decades but appeared spectacularly on the radar in North America during the 1999 World Trade Organization ministerial meeting in Seattle.

Police cracked down hard and a peaceful march and blockade by 50,000 anti-globalization protesters degenerated in a cloud of tear-gas and pepper-spray.

With no structure and no purported leaders, they're maddeningly hard for authorities to pin down.

"What we know is what we pulled up on the Internet," said a spokesperson with the RCMP-led Olympic Integrated Security Unit.

Funny, but close to the truth, says Michel Juneau-Katsuya, an anti-terrorism expert and former member of the Canadian Security Intelligence Service.

There's a lot about Black Bloc actions on the web but little to help track individuals. They may belong to other groups or none at all, and need only show up at protests in the basic uniform -- black clothing and a scarf or balaclava -- and ready for action.

"Not having any sort of structure to that kind of organization makes it very difficult because basically you're just trying to chase dust," Juneau-Katsuya said from Ottawa.

He said police tried to infiltrate the group at the 2001 Summit of the Americas in Quebec but the undercover officers' age and police-issue shoes gave them away.

The Black Bloc has an uneasy relationship with those it ends up marching beside. Some of the groups opposing the Vancouver Winter Olympics won't disavow the violent actions they took on Saturday.

Adherents say it gives their causes an attention-getting edge but others worry it muddies their message and alienates would-be supporters among the public.

The Black Bloc strategy is simple: Show up at demonstrations and attack the prominent symbols of capitalism. Then hope the police react, or over-react, while they shed their black clothes and melt into the crowd.

In Seattle, it was McDonald's and Nike. In Vancouver, Black Bloc members smashed windows at Olympic sponsor HBC's downtown store displaying Games merchandise and spray-painted the anarchist circle-A symbol on at least one bus and city vehicle.

A Black Bloc contingent that was part of a march by about 2,000 people Friday evening that coincided with the Games' opening ceremonies tossed things at a police line blocking access to B.C. Place stadium.

Police pushed back but the scuffle failed to spark a Seattle-style melee.

Saturday's vandalism brought a stiffer response, including seven arrests, though protest organizers said none were part of the Black Bloc action.

Supporters of aggressive tactics call them successful because they force a thickening of security around big events and further expose what they see as a police state.

But a U.S. anarchist in Vancouver blogged that on Saturday bloc activists were getting heckled by bystanders themselves.

The protest also exposed cracks in the anti-Olympic movement.

David Eby, executive director of the B.C. Civil Liberties Association, said he was "sickened" by images of Black Bloc members smashing windows and tossing newspaper boxes into the streets.

Eby, who's been outspoken against what he considers police intimidation of Olympic dissidents, said protesters were prepared for civil disobedience, such as sit-down demonstration to block an intersection.

Eby said there was a hint that the protest would turn violent when his group was asked by a faction of the protesters not to send legal observers to the march, saying they were worried they'd been infiltrated by police and could have their video documentation used as evidence in court.

Anti-Olympic protest organizers spun Saturday's actions as non-violent attacks on inanimate objects by those who think the more peaceful message is being ignored.

"It's a situation where people feel frustrated," said Melissa Westergard-Thorpe of the Olympic Resistance Network.

While stopping short of endorsing the vandalism, she said her group supports "a diversity of tactics."

Eby rejects the explanation.

"As far as our organization is concerned, property destruction is violence," he said.

The Pivot Legal Society, as an advocate for residents of the poverty-stricken Downtown Eastside, has had bitter disputes with Vancouver police in the past but reached a deal with them to unfurl a huge banner on a bridge adjacent to the athletes village Monday as part of a tent-city protest against homelessness.

Juneau-Katsuya said police have appeared to show a lot of restraint in the face of Black Bloc provocation. That may change if they see signs of anything close to what happened Saturday.

"I'm predicting that we might see a little bit more prompt reaction from the police," he said.