'No Pants' Travelers Swarm San Francisco

SAN FRANCISCO (CBS SF) - The annual flash mob tradition of "No Pants Subway Ride" returned Sunday. The day when fun-loving participants from all over the Bay Area knowingly and willfully travel on BART and/or MUNI clad in their skivvies, sans pants.

PHOTOS: Worldwide 'No Pants' Subway Ride

The event, hosted by prank collective "Improv Everywhere," is held in over 15 countries around the world on the same day. The collective suggests wearing a normal top with colorful or fun underwear. Nudity is not encouraged but carrying money for mass transit like a Clipper Card is required. Participants are required to travel with a straight face and act as if nothing is out of the ordinary.

Participants in the San Francisco Bay Area traveled on BART/MUNI with no pants to the Montgomery Station by 1:26pm. Then follow the crowd to the final destination at Yerba Buena Gardens by 1:50pm for a group photo.

In the past, pants-free people were encouraged to walk into stores, bars, bathrooms and malls to get reactions from people, all the while, acting as if they are unaware of people looking at them curiously. This annual event's Facebook page date back to 2010 and overall is intended on being a light-hearted display.

Last year's viral event managed to draw in over 600 pants-free people and is expected to exponentially grow this year.