The organizers of San Francisco’s annual pants-free BART/Muni ride asked participants to heed a single warning ahead of Sunday’s festivities:

“Don’t make this event like SantaCon.”

Originally staged by the New York City collective Improv Everywhere as a casual prank, the unusual display has grown into a worldwide phenomenon that’s been parading across platforms and bumping into unsuspecting commuters since 2001.

It's San Francisco’s 12th year getting in on the action. Over 600 participants converged at the Daly City and MacArthur BART stations Sunday afternoon, given instructions to bring their backpacks and Clipper cards and otherwise don “normal SF winter clothes — whatever that means.”

No nudity is permitted. In lieu of pants, those daring enough to partake were advised to wear “interesting undies and colorful socks.” If confronted during the journey by a curious kid (or maybe an angry parent), they had to keep a straight face and act completely normal.

AT LAST YEAR’S EVENT: Pantsless people take over BART, puzzle passengers

BART director Janice Li told SFGATE that while she wasn’t familiar with the event, she was happy to see the boost in weekend ridership in spite of decreased BART service resulting from critical maintenance work.

“The fact that people are riding BART on Sunday is great and I will keep finding ways to improve service, but generally with clothing on,” said Li.

However, she did have one question for this year’s participants: “Why January?! So cold!”

Riders disembarked at San Francisco City Hall, where the event was capped off by a no-pants-necessary afterparty.

“This is weird Bay Area at its finest and I hope the Bay Area stays weird,” said Li.

Amanda Bartlett is an SFGATE associate digital reporter. Email: amanda.bartlett@sfgate.com | Twitter: @byabartlett