Today's Northern Hemisphere ride, which coincides with events in other U.S. cities and in Canada and Europe, was scheduled to start at noon, and people started gathering at Justin Herman Plaza at 11 a.m.
According to an event organizer, who asked to go by "Nud" when speaking to the media, the ride intended to draw attention to the over-consumption of oil and reliance on cars, with bare-bottomed riders expected to pass through popular city streets along the waterfront, Market Street and into the Castro District and Golden Gate Park.
"We are trying to get us less reliant on cars," said the San Francisco resident who has participated in the event for the past five years.
He said the ride attracts many activists who live by the motto "get involved, get active," while other participants strip down to cross "naked bike trip" off a bucket list.
Riders behind the plaza's fountain were disrobing, slathering on sunscreen and buckling up helmets for the trek.
Berkeley resident Jasmin Bradley, 21, was completely bare, save for her flip-flops, as she geared up for her first naked ride with her 26-year-old friend Oakland resident Shelagh Jackson, who participated last year.
Both do not have driver's licenses and rely on their bicycles and public transportation to get around. Today's ride "makes people look," Jackson said, because the nude riders planned to shout slogans such as "Burn fat, not oil."
For Jim Zaun, 62, the ride is a three-year tradition that brings him to San Francisco from his Boulder Creek home in Santa Cruz County.
The engineer, who said he is often naked at home and has visited a local nudist colony, is the creator of the ride's logo of a bare-breasted woman on a bicycle with the Golden Gate Bridge behind her, which he drew three years ago.
"It looked like (the ride) needed more of an official presence," he said. "It shows were not a group of perverts ... people realize we are projecting a message."
He said passing pedestrians often cheer and whistle, while a small percentage is offended.
A growing mix of tourists, locals and gawkers on the plaza's northern grassy slopes were snapping pictures, pointing at the scantily clad crowd, and taking in the very San Francisco atmosphere.
Richie and Randi Goldstein, 66 and 62, respectively, a couple who lives near the plaza, said they saw some tourists who looked shocked at the public display of nudity and made sure to tell them "Welcome to San Francisco."
Just after noon, the bare riders assembled for a group photo in front of the fountain and cheered, as they readied to head off on their route.