Monthly worldwide event in which cyclists temporarily liberate major urban roads from cars. The London rides started in April 1994.

It isn’t just for cyclists – sometimes there are wheelchair users, skateboarders, roller bladers and skaters, and other self-propelled people. Cycle sound systems often accompany the ride, creating a colourful, party atmosphere that welcomes riders of all ages and abilities.

The critical mass happens on the last Friday of every month (regardless of weather, date, police, Godzilla coming up the Thames or lack of cake).