Those little dungaree darlings who glue themselves to motorways for entertainment, and throw soup at famous paintings in the UK, should be very grateful that they don't live in France.
In the UK the Cops simply stand-by in large groups and watch, but South of La Manche they have a very different way of dealing with people who disrupt the smooth running of society.
Any slight hint of anarchy and out come the CRS (Compagnies Républicaines de Sécurité) to give them a good walloping. The CRS are not the ordinary Gendarmes, they are a more sinister group who are often seen in groups, sitting inside dark military vehicles by the road side in the larger French cities. They are fully kitted, and ready to pounce!
As I live in a quiet corner of little-known France, we have no CRS presence, so I have never personally seen them in action, but I've seen plenty of reports and film.
It isn't surprising that experienced French demonstrators/rioters often wear motorbike crash-helmets. They need them!