A Black Bloc protester smashes a department store window in Vancouver during the Winter Olympics. The No. 1 target of the Black Bloc at the G20 in Toronto is expected to be the tall mesh security fence surrounding the centre where the leaders will meet. (CBC)

When more than 5,000 police officers take to the streets of Toronto during the G20 summit meetings June 26-27, their biggest worry may be militants whose tactics have earned them the name Black Bloc.

The self-described anarchists promote violent confrontation with the authorities, which is why they may be a major concern at the G20, according to CBC News security correspondent Bill Gillespie.

Black Bloc protesters wear ski masks or balaclavas to conceal their identity, rarely do interviews, and on their websites rail against capitalism and banks.

They allege that exploitation of Third World countries by the rich G20 nations causes poverty and death.

So, according to Black Bloc logic, throwing rocks at the police is a justifiable, even moral, response to the violent consequences of G20 policies.

Canadian security analyst John Thompson doesn't buy it.

"They've been described as activists. Forget it. The cause is irrelevant. What they really are is confrontation junkies," Thompson said.

Typically, a tightly knit phalanx of Black Bloc members will insert themselves into a large, peaceful protest march. When the march passes a bank, one of them might run out of the group and throw a brick through a window.

The brick thrower then quickly runs back to his masked, black-clad comrades, leaving the police unsure whom to arrest.

The No. 1 target of the Black Bloc in Toronto is expected to be the tall mesh security fence surrounding the centre where the G20 leaders will meet, said the CBC's Gillespie.

Protesters using Black Bloc tactics were active during the Winter Olympics in Vancouver. Three were arrested after windows were broken at a Hudson's Bay Company store and cars were vandalized on West Georgia Street.