Bare legs and briefs filled train cars from Sydney to New York Sunday as passengers traveled trouserless -- provoking laughs and perplexed looks -- for the 13th annual "No Pants Subway Ride."

Commuters in some 60 cities braved public transportation in their undies for the stunt, which has gone global since its first staging by US group Improv Everywhere in New York in 2002.

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

"It is just about fun, and providing a laugh and a smile," said Charlie Todd, who created the event 12 years ago when just seven people took part.

Act "normal. Remember to keep a straight face," Todd instructed participants in the Big Apple -- some 3,000 to 4,000 people this year aged three months to 71.

"If somebody tells you you have no pants, just say you forgot, and yes it's cold, yes it's unfortunate," he said.

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 in an effort to up the impact on bystanders.

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

Seven New York subway stops were designated for this year's stunt, with the group planning to reconvene ultimately at Union Square in Manhattan.

"It's a bit chilly, but it is fun," said Pedro, in red underwear and socks, wearing a hat and carrying a briefcase, as he stood on a subway platform.

A woman in her thirties donning briefs and talking on her cell phone prompted amused glances from passersby, with one older woman sporting a perplexed stare.

In Paris, the meeting point was the Charles de Gaulle-Etoile metro stop, with pants-free passengers headed all the way to Bastille. More than 1,200 people had signed up ahead of time via Facebook.

Other events were smaller, like in the Argentine capital Buenos Aires, where a handful of young people, mainly men in suits, ties -- and brightly-colored flowery boxers -- drew laughs.

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.

"Insist that it is a coincidence that others also forgot their trousers," the organizers told participants, who were also forbidden from speaking to one another.

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.

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 internationalized now," said 22-year-old Huang Li, wearing blue underwear decorated with dots.

But "people say I'm a psycho," she added.