Personally I do agree with the views of anti-capitalist protests, and that you do need police at a demo to make people sit up and take notice, up to a point. This doesn't mean you have to trash things. (By the way leprechaun, I don't think that was 'a rare moment of candor' saying that you need the police at a protest - everybody knows that, it's common knowledge, and whoever you spoke to probably presumed you knew it too)
What I don't agree with is using violence to make a point. It's not big and it's not clever, and it hurts (not just physically) those who do not deserve to be hurt. Anyone who has been at any anti-capitalist protest will most likely tell you the violence was initiated by a small anarchist group, and this is not about anti-capitalism; it's explicitly about causing trouble. There are many many people out there who agree with the aims but not with the means, and yet most of the time these voices don't get heard because the media are too busy concentrating on the acts of the evil anti-capitalists, tarring all with the same brush. It's understandable (I know which makes the better headline) but deeply unfair and deeply biased.
And it justifies people in bringing out every stereotype they can think of to criticise anyone who dares suggest another way of living, becuase after all these people are all just terrorists, aren't they? (medical marijuana ground by soy-bean/hemp coffee-grinder anyone? sandal-wearing muesli-eating Guardian readers? dreadlocked commie hippies who don't wash?) So we can say (and do) what we like to them, if they're terrorists?