Roller Paris

Every Friday evening (as long as the weather permits), leaving from the Tour Montparnasse at 22h, huge crowds of people gather to spend 3 hours...rollerskating around Paris.
The streets are closed off, the police follow at a safe distance, and there are numerous volunteer helpers for those who get tired, lost, or who just want a chat. All this security is absolutely necessary, because numbers can sometimes rise to 15000 participants, of all backgrounds and ages. The event itself is free, set up initially by a small group of "patineurs" (skaters), which gradually gathered momentum and is now a veritable association. Skates- or blades- can be rented, or old pairs dug out from the backs of cupboards. There is also a Sunday version, run by a different association, called Rollers & Coquillages.
NB: Since I am not into exercise of any kind, don't ask me any more questions about this :)