The truth is that the organizers of the event exercised reasonable control over most of it but could not do so over the various fringe groups. Possibly they could do never have done so though some of their slogans of demand may well have acted in an inflammatory manner. Nevertheless, organizers of rallies and demonstrations do have a duty of care and control. It is no good blaming other people. Those in charge have to take responsibility.
All that is by the way. What I object to is the definition of those groups as anarchists. An "anarchist", according to the OED is one who advocates anarchy, which is absence of government and disorder. Some anarchists, for instance, followers of Kropotkin rather than Bakunin, are and were against violence, maintaining that the disorder that would arise from absence of government would create its own, much better order. Bakunin thought violence was essential.
There are various branches of anarchism. Anarcho-syndicalism concentrates on the labour movement, believing in co-operative ownership and production; anarcho-capitalism believes in the supremacy of individual ownership and the free market.
One can discuss these matters at length and, indeed, some people do. What they all have in common is their disdain and dislike for the state. Indeed, they all share a desire to do away with the state.
The rioters in London on Saturday shared the general view of the demonstrators of "no cuts", big state, high taxation, disdain for private enterprise and free economic activity of any kind. They were not, by any definition, anarchists. It is time to think of another name for them. How about "socialists"?