It's worth keeping in mind that not all anarchists are anarchists. Some are undercover police. This has been proven to be the case in incidents at G20, and as another case highlighted by Mayomick elsewhere, also grassroots anarchist organisations in the UK. The official line of course will be that such infiltration is merely for the purpose of observation. A conspiracy theorist might have different ideas. Especially where violence is in fact instigated by men wearing the same boots as their uniformed colleagues, and then protected by them when the crowd identifies them as not being legitimate demonstrators.