No Pants Subway Ride catching on around the globe (PHOTOS)

Trains and trams in major cities around the world were awash with bare legs and briefs on Sunday as pranksters traveled trouserless for the world's 13th annual "No Pants Subway Ride."

Commuters from Beijing to Bangalore and beyond braved public transport without blue jeans in the stunt, which has gone global since its first staging by US group Improv Everywhere in New York in 2002.

Twelve years ago, only seven people took part in the inaugural event, but now thousands in major capital cities around the globe participate in what organizers call a "celebration of silliness."

The premise is simple: participants convene on a given transport route on a designated day every year without trousers on, and ride the rails (or road) for shock value and laughs.

Underpants must be worn and, although flashy designs are allowed, organizers prefer those involved to look — at least from the waist-up — as though they are going about their daily lives, to increase the impact on bystanders.

Uniforms and business suits are encouraged to amplify the lower-half effect as are props — bicycles, prams, shopping bags or a briefcase.

Participants are also forbidden from speaking to one another and are instructed to bring "any activity you might normally perform on the train: newspaper, book, sewing kit."

"If questioned, you do not know any of the other pant-less riders. Tell folks that you 'forgot to wear pants,'" organizers told participants ahead of the Sydney ride.

"Insist that it is a coincidence that others also forgot their trousers. Be nice, friendly and remain calm."