Police are concerned that the biggest threat to the G20 summit meetings in Toronto this month - despite the high security fencing and strict access control - will be militant protestors known as Black Blocs.

According to CBCNews, the protestors are not afraid of violent confrontation with authorities and their efforts to conceal their identities make them particularly dangerous.

Commenting on their methods, Canadian security analyst John Thompson told the news provider: "They've been described as activists. Forget it. The cause is irrelevant. What they really are is confrontation junkies."

The paper claims that Black Bloc protestors often use the cover of legitimate protests to avoid arrest.

It added some protestors were using Black Bloc techniques at the recent Winter Olympics in Vancouver.

Toronto Police superintendent Tom Russell recently claimed that while the high security fencing being erected around the city was an inconvenience to many local businesses and residents, it was absolutely necessary.ADNFCR-3337-ID-19840628-ADNFCR