Every month, cyclists in London and other cities get together for "Critical Mass", a chance to ride around the city streets free from cars and lorries, simply by getting enough bikes out there that cars and lorries have no room to go anywhere. The London one attracts around 1000 people in summer and is growing.

Central London Critical Mass meets on the last friday of every month at the Southern end of Waterloo Bridge, in front of the South Bank Centre. People start arriving at about 6:00, and the mass leaves some time soon after. Rides generally take in circuits of central London sights - the palace, Trafalgar Square, Parliament, Hyde Park, Piccadilly .....