The ninth-annual No Pants! Subway Ride is scheduled for Sunday afternoon and is being organized by Improv Everywhere, a New York-based organization.
The group, which pulls pranks and holds impromptu gatherings in various places, says on its Web site that the goal is to "bring excitement to otherwise unexciting locales and give strangers a story they can tell for the rest of their lives."
The prank artists plan on acting normally, and not interacting with any of the other pant-less riders in an attempt to confuse other passengers on the train who are not in on the joke.
Along with New York City and San Francisco, similar events are planned in Atlanta, Boston, Chicago, Cleveland, Dallas, Denver, Honolulu, Los Angeles, Philadelphia, Phoenix, Pittsburgh, Salt Lake City, San Diego, Seattle, and Washington, D.C.
The San Francisco event is scheduled to begin at about 1:30 p.m. Sunday. Riders will get on BART trains from the East Bay and the Peninsula and travel toward San Francisco.
The trains will both arrive at the Embarcadero station around 2:15 p.m.
Vivek Ramachandran, an organizer for the San Francisco event, said more than 300 people rode BART without pants at last year's event, and he expected more people this year.
He said when the group got onto the train they were met with "surprised responses, some laughter, and most people were very inquisitive."
He said while most people wore boxers, some had "somewhat questionable apparel."
BART spokesman Linton Johnson said event participants should be careful not to break any laws with their choice in clothing, or lack thereof.
"There are little kids and little grannies on BART," Johnson said. "It's called indecent exposure, so keep your pants on, or you might get arrested in your undies."