You need permission for a demo in France. An “authorisation préfectorale” is needed for any event in a public place whether it’s music, a sporting event such as a bike race, anti mariage pour tous or the CGT moaning. However, keep the numbers down and you’re just covered by the laws of what you can and can’t do in public.
That’s the theory but when the gouvernement really upsets people then cobbles start flying regardless.
A swimmer interupting race would get arrested, locked up for the duration of whatever he was protesting about, then assuming there was no “outrage à agent”, kicked out with a warning or a modest fine to pay.