There's been a lot of crying about "thugs and anarchists" in Toronto. I live about 4 blocks from where some of the vandalism occurred, though I wasn't there at the time.
As best I can tell, what happened is that for about an hour, the Black Bloc protesters clearly and visibly prepared for action, with both the police and other, non-violent protesters able to see they were doing so. The number of Black Bloc vandals seems to have been between 50 to 100, certainly not more than 200. (The police had 20,000 men.)
The police actually withdrew, leaving behind police cars for the Black Block to torch. Which they then did. The Black Bloc then proceeded up Yonge street (the main north/south street in downtown Toronto), vandalizing as they went, and eventually many headed over to Queen's Park, the Provincial capital. Two hours after the first violence, the police finally take action, ensuring that there are plenty of videos of police cars burning and vandalism that would not have occurred if they had taken action earlier.
According to the police, rather than confront a maximum of 200 protesters, they withdrew behind the barrier around the G20 meetings and let them vandalize downtown Toronto for 2 hours.
At the end of the day the people who matter never even saw any protests and the 1 billion dollar police presence and suspension of civil liberties was "justified" by vandalism and burning police cars.
Simply put, the police decided that they couldn't spare, say, 2,000 out of their 20,000 men to stop 200 vandals. This was a deliberate decision to allow downtown to be vandalized.
I leave it as an exercise for readers to decide if this was a matter of incompetence, or if it was a deliberate strategy. And if it was deliberate strategy, just what they were trying to accomplish with their strategy.
Of course, along the way Canadian Civil Liberties observers were arrested as well, and protesters were not allowed to see lawyers.
I am ashamed to be Canadian today, and I am ashamed of my governments, at all levels.