The Bay to Breakers. is an annual 12k footrace which takes place in San Francisco. A ton of participants and race crashers alike (60 thousand + some) walk the route behind the runners. Many dress in elaborate costumes or, though not technically allowed, wear nothing at all (except footwear). Those that do wear clothes choose their most suggestive outfits, thus lending a party atmosphere to the event.
Golden Gate Park, San Francisco.