They reckon it's the most fun you can have on a bike, and they should know - they've been doing it for the last ten years.

Critical Mass London started back in April 1994,
and has been described as an 'unorganised coincidence'.

It happens when a lot of cyclists happen to be in the same place at the same time and decide to cycle the same way together for a while, with a shared wish to see less car-dominated cities and more people cycling.

The first London Critical Mass had a group of around 50 riders, but Summer masses in London have attracted well over a thousand cyclists in the past - they usually ride on the last Friday of every month.

Each one is different and follows no set route, with the direction being spontaneously chosen as people cycle along. Anyone is free to join or leave the ride as it pedals along.

This Friday marks their tenth anniversary, so they're planning an extra special party on a bike to celebrate, meeting at 6pm on the South Bank.

The ride could last a couple of hours, depending on the weather, and usually ends in a place of interest somewhere in central london.

Everyone's welcome, whether you're on a bike, skates, roller blades or a pogo stick! And they always aim to be peaceful, safe and fun!

For more information visit the Critical Mass website ( http://www.criticalmasslondon.org.uk).