Hundreds of people flocked to the London Underground cheekily flashing their flash as part of the unusual annual event.

The No Trousers Tube Ride came under the international event known as No Pants Day, where metro lines around the world were taken over by bare-legged crowds.

The umbrella event is organised by practical joke group Improv Everywhere, and started as the No Pants Subway Ride in New York 11 years ago.

It has now spread to 60 cities in 27 countries including Washington, Mexico City, Toronto, Madrid and Tel Aviv, with the help of Facebook to muster more than 16,000 participants.

The flesh-baring groups converged, then performed flash mob-type stunts around the globe, surprising unsuspecting by-standers as they stripped off on public transport.

In Madrid, the police were unimpressed as they stopped people from entering the starting station.

Organiser Dan Becherano said the purpose of No Pants Day was purely to entertain people.

“The purpose of flash mobs is to show how we can get together without knowing each other and work as teams without really having seen each other ahead of time.

“The most important thing will be the victims – the reaction of people who aren’t involved.”