We do have that nice law that makes you responsible to keep a protest safe - if you cannot gurantee for the protestors safety (and that they are themselves not a safety risk), you cannot protest. It's as simple as that.

When we protest on the streets, we have to declare this protest a few days in advance and tell where the protest will be staged. If the police thinks that the protest is causing too much problems, e.g. traffic-wise, they can either forbid it or make us use another route. Both decisions can be overturned by the courts, but usually, they handle this rather sanely and with civil rights in mind. They do forbid fascist protests in immigrant quarters, for example.

The protest route is closed down for traffic for the alotted time of the protest, and detours are set in effect. It is also mentioned in the bihourly traffic radio when it is likely to affect a significant amount of drivers.

I have never heard of a police-approved protest staged on streetworthy vehicles (there have been a few wild protests by farmers who used their tractors, and who were removed quickly), but I suppose even then traffic laws would apply, which would make every single person in this picture liable and more likely to lose their drivers license (and a hefty amount of cash).