Speaking on the eve of a student day of action, the officer in charge of monitoring "domestic extremism" in the UK said his officers' focus was on the "fringe" where protest "spills over" into violence and disorder.
In recent days, police have selectively removed some barriers on the fringes of protest sites.
By the time the police-station siege ended, it had turned from a fringe-party protest to a more general uprising against the government.
About 14 percent of respondents said they were inclined to support fringe parties and protest movements that disparage the mainstream political class.
The leader of France's oldest and most powerful union, the Confédération Générale du Travail (CGT), said the hospital attack was "scandalous" and "completely unacceptable", and blamed hooligan elements on the fringe of the protest.
In March, in contrast, London's police had been criticised for being too lax in responding to vandalism on the fringes of a protest against government cuts.
Regardless of fringe rucks, these protests are more likely to lay the ground for wider public and industrial campaigns than frighten them off.
"The whole concept of fringe began as a protest to an over-normalized theater culture," said David Ortolano, its executive producer.
"Some of the politicians who are supporting the war want to be protested by fringe groups.
