"Protesters smashed windows and WAVED ANARCHIST FLAGS from the roof of the building housing the Conservative party headquarters as the fringe of a vast rally against university funding cuts turned violent.
[...] PROTESTERS WHO BROKE OFF FROM THE MAIN ROUTE OF THE MARCH occupied the lobby of the building at 30 Millbank, in central London, where police wielding batons clashed with a crowd hurling placard sticks, eggs and bottles. ACTIVISTS WHO HAD MASKED THEIR FACES WITH SCARVES traded punches with police to chants of "Tory scum". The exchanges were recorded by broadcasters' helicopter-mounted cameras and protesters' mobile phones." (Guardian)

So a group of anarchists and not students (concealed faces, waving anarchist flags, breaking off together from the route) quite evidently had this violence preplanned.
The question is how in future genuine protesters can identify such small groups and prevent them from hijacking otherwise peaceful demonstrations?