There were giggles, gasps and glares on the MTR yesterday as passengers boarded the trains having seemingly forgotten to get fully dressed - and not yet realised.
Scores of people took part in the city's second No Pants Subway Ride yesterday in an annual "celebration of silliness" aimed at causing "scenes of chaos and joy in public spaces".
The event was started by New York-based theatre group and "prank collective" Improv Everywhere in 2002.
At least 40 men and women gathered, fully dressed, at the IFC Mall in Central before heading underground and splitting into groups and boarding a train. At Admiralty, they exited the carriages and the trousers came off.
For more than 90 minutes, MTR commuters from Central to North Point to Prince Edward were treated to a bizarre display of underwear. The half-dressed passengers gave nothing away as they journeyed nonchalantly around Hong Kong and Kowloon before ending the event at Hong Kong Station.
One man, coolly listening to music, showed off his Homer Simpson boxers, while another man dressed in a smart shirt, tie and black boxers read a book silently next to a baffled woman and her granddaughter.
One woman, seemingly absorbed in her iPhone screen, was clad in a bright orange pair of knickers with the words "HANDS OFF" emblazoned on the back.
Amused passengers giggled and took photos. Others stared in bemusement as they tried to comprehend what was going on.
But according to Improv Everywhere founder Charlie Todd, nothing is going on. He refers to the event as having "no agenda apart from a desire to make others laugh and smile".
One of the co-organisers of yesterday's event, Enoch Chan, said: "We aim to spread this Western prank culture to relatively 'traditional' Hong Kong … Many commuters were astonished and thought our actions were morally unacceptable."
The team were disappointed that only about 20 per cent of the 200 people who signed up on Facebook actually turned up on the day.
An MTR spokesman said the company had not received any complaints from passengers yet. "We trust that passengers will not do anything that would cause inconvenience to others," he said.
Watch: Legs bared in Hong Kong for annual No Pants commute