Sydney (AFP) - Trains and trams in cities around the world were awash with bare legs and briefs on Sunday as pranksters travelled trouserless for the 13th annual "No Pants Subway Ride".Commuters in Sydney, Beijing and Hong Kong braved public transport without trousers 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 organisers 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, organisers 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'," organisers 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."
In Sydney, a small but dedicated group dropped their trousers on the city circle loop train, packed with bemused weekend sightseers on a busy summer's day, then switched routes for a return trip to Bondi.
There was a no trousers tram ride in southern Melbourne and Adelaide and people also went pants-less for a train trip in northern Brisbane.
On a busy Hong Kong subway train around 40 people, including Australian Bess Hepworth and her 18-month-old son, joined in the fun.
"It's the first time he will be publicly taking off his pants, although he does it regularly already," the 37-year-old said.
In Beijing a handful of trouserless people took to the Chinese capital's fast-growing subway, drawing confused glances from onlookers.
"I want to show that (Chinese people) are internationalised now," said 22-year-old Huang Li, wearing blue underwear decorated with dots. "People say I'm a psycho," she added.
Improv Everywhere expects some 4,000 participants in this year's New York event, with more than 60 other cities taking part including Paris and London.