Hundreds of subway riders all over the world on Sunday travelled without their trousers on. Declared ‘No Pants Subway Ride’ Day, the occasion is planned for winter every year, and sees commuters don all their regular warm clothes on top, but just their underpants at the bottom.

The event began in 2002 with seven members of the improv comedy group Improv Everywhere rode the subway in New York City without their pants. Since then, hundreds have joined in.

'The idea behind No Pants is simple: Random passengers board a subway car at separate stops in the middle of winter without pants. The participants behave as if they do not know each other, and they all wear winter coats, hats, scarves, and gloves. The only unusual thing is their lack of pants,’ reads a description on Improv Everywhere's website.

According to CNN, the event has spread far beyond New York City, with participants going bare-legged in Warsaw, London, Berlin, Prague and Bucharest, among many other cities. The exact number of pants-less commuters has not been determined.