These "critical mass" rides happen all over the world, in over 300 cities, and have frequently resulted in debates regarding their legality.
This Wikipedia articles documents such legal cases in San Francisco (which is regarded as the birth-place of the event), New York, and the current one in London.
Typically, the rides start at 6pm on the last Friday of every month, always starting in the same place, but normally then taking different routes around the city and are essentially without an organizer or leader. Their main purpose is to promote cycling and any alternatives to cars and congestion. Because so many riders take part, they tend to disrupt traffic flow thus bringing attention to themselves from the public and the authorities, which is their goal.
Websites with information about critical mass rides around the world are
critical-mass.org and
critical-mass.info. The former site lists such an event in Zurich, but the link does not work. The latter site lists events in Basel,
Geneva, Neuchatel, and
Winterthur, with working links for two of these places, which I have copied.