There is no need to provoke a riot at the G8/G20 meetings either. The "protesters" have established a rich history spanning a couple of decades now of forcing their demonstrations into pre-planned violent events (all under the guise of helping the planet).
These folks come prepared Cliffy and if you doubt it, just cruise the blogosphere and eavesdrop on what's being stated.
It's easy to post video that lacks the full context, especially when the shots are taken by someone with an agenda... Fact is, there are 2 sides to all stories and to assess cause-effect from only one angle is just incomplete.
The so called Black bloc are all in uniform, many carry police issue clubs and wear police issue boots. Police have been infiltrating dissenting groups for over 50 years to incite violence. There are many documented cases, one that comes to mind was an RCMP agent who started a white supremist group and incited violence in Alberta. You may remember that case.
When I lived in Montreal, the cops were always starting riots. One of the most notorious was the La Press strike around 1969.
And going back even farther, there was the attacks on unions in the 30s and 40s.
You want to scan the blogosphere you will find agent provocateurs writing stuff too to further discredit dissenters. Nothing is as it seems out there. The TO G8/G20 was a training exercise. Expect the next one to be worse.