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.
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http://crooksandliars.com/ian-welsh/police-allow-vandals-run-amok-2-hours-g2