: Unless it's in response to immediate violence, non-violence is the only viable tactic for demonstrations. Do you know what happens when demonstrators turn violent? 1) They get their asses kicked by superior police forces, and worse, 2) They lose the support of mainstream society, and rightfully so.
Unfortunately, this gives the police carte blanche to behave as badly as they like to the demonstrators in the knowledge that any violence which takes place will be blamed on the protestors; it's rather like the old defence against rape; "they were asking for it, going around like that".
When it comes down to it, riot police are so heavily armed that they can start a fight with comparative impunity.
On June the 18th 1999, the police were wandering around on an all-day power trip; they were trying to confine the protestors to a ridiculously small area (and you know how people behave when they feel hemmed in).
Even this didn't manage to turn people nasty; in the end, they ran two people down with riot vans and prevented an ambulance getting to one of them (they said that the protestors "were a threat to the safety of the ambulance staff"; and delayed the ambulance getting to the woman they'd run over) for 25 minutes; she was crying and moaning; and that turned the crowd ugly; and as soon as the crowd turned ugly, the police decided it was time to wade in full strength and charge them with the cavalry.
Protestors very rarely indulge in violence for no cause; especially the hippy-green anti-road lot. However, even if you only react with violence to immediate violence, you will still get bad press for resisting at all.
In my qualified experience, the police were intent on premeditated violence yesterday; as they were on 18/6/99, as they were on 13/4/97 and as they were on 9/10/94; I speak as an eyewitness at all of the above dates.
I believe that the police in question were ordered to create the conditions necessary for a riot to further the political ends of their political masters; and that it will happen again; it seems to be the standard tactic used to discredit a mass movement.