Bare legs and briefs made up for the trains and trams in Australia as pranksters travelled trouserless for the world's 13th annual "No Pants Subway Ride".
Since its first staging by US group Improv Everywhere in New York in 2002 with just seven people, No Pants Subway Ride has gone global, with thousands of people now joining the stunt in major capital cities across the globe.
The premise is simple: participants convene on a given transport route on January 13 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.
Improv Everywhere expects some 4,000 participants in this year's New York event, with more than 60 other cities also taking part including Hong Kong, Delhi, Paris and London.
It describes the event as a "celebration of silliness".