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Groups of half-naked Londoners took to the Tube en masse as part of this year's No Trousers on the Tube Day.

More than 400 people were set to attend the ninth annual event on Sunday, which urges people to brave the cold weather barelegged.

It was part of the worldwide No Pants Subway Ride which began as a small prank in New York in 2002 and has since spread to over 60 cities around the globe including Tokyo, Jerusalem and Moscow.

Despite a bitter cold snap that could bring subzero temperatures overnight, travellers embraced the gimmick, where they were encouraged to keep a straight face and "sit in the car as they normally would".

The group, led by organisers The Stiff Upper Lip Society, met at Westminster at 3pm before undertaking an hour and a half of riding trains, followed by an after-party.

People of all ages got involved in the event, in which they were wrapped up for winter on top and bare below.

Images shared on social media showed participants sporting brightly coloured underwear in the spirit of the event.

Groups filled carriages of the Underground with their exposed legs as they sat casually on the train.

One traveller commented on Twitter: "So it appears a lot of people are having an issue with their washing machines."

The Facebook event information read: “Participants will be organised into groups, and will walk to a nearby station.

“When told to by your group leader, remove your trousers, and put them into your backpack.

“For the remainder of the journey, sit in the car as you normally would. Read a magazine or whatever you would normally do.”

The organisers added: “You can wear fun underwear if you like, but nothing that screams out, 'I wore this because I’m doing a silly stunt.'

No Trousers Tube Ride 12 show all No Trousers Tube Ride 1/12 Some passengers drew more attention to themselves than others EPA 2/12 Hundreds of passengers took part in the bizarre annual event EPA 3/12 Londoners on trains all over the capital saw far more than they wished to EPA 4/12 Just a normal Tube journey... but no trousers EPA 5/12 A group of brave passengers is snapped at Elephant and Castle EPA 6/12 Chilly EPA 7/12 Travel in style EPA 8/12 Passenger frolick on London's Bakerloo line EPA 9/12 The phenomenon was born after seven friends in the US boarded the New York subway without their trousers for a joke EPA 10/12 Mind the gap EPA 11/12 Nothing to see here EPA 12/12 Passengers leg go of their inhibitions EPA 1/12 Some passengers drew more attention to themselves than others EPA 2/12 Hundreds of passengers took part in the bizarre annual event EPA 3/12 Londoners on trains all over the capital saw far more than they wished to EPA 4/12 Just a normal Tube journey... but no trousers EPA 5/12 A group of brave passengers is snapped at Elephant and Castle EPA 6/12 Chilly EPA 7/12 Travel in style EPA 8/12 Passenger frolick on London's Bakerloo line EPA 9/12 The phenomenon was born after seven friends in the US boarded the New York subway without their trousers for a joke EPA 10/12 Mind the gap EPA 11/12 Nothing to see here EPA 12/12 Passengers leg go of their inhibitions EPA

"Wear two pairs of underwear if it makes you feel more comfortable.

"Don’t wear a thong or anything else that might offend people.

“Our aim is to make people laugh, not p*** them off.”

Last year 300 Londoners took part in the stunt, which always takes place in early January.