PHILADELPHIA -- A little after 5 p.m. Saturday, crowds began to gather at Rittenhouse Square. Occasionally, a car would stop to ask what they were waiting for.

For those in the know, it was a much-anticipated event. For those unaware, it came as a bit of a surprise when hundreds of naked bicyclists took to the streets of Philadelphia.

The 9th Annual Philly Naked Bike Ride kicked off with a pre-party at Glendinning Rock Garden, a small park just across the Schuylkill River from the Philadelphia Zoo. Here, the participants had the opportunity to have their bodies painted with artwork to help cover -- or highlight -- certain areas, or haphazardly have paint, glitter and colored "dust" applied as they lie on a sheet next to friends or complete strangers.

PNBR, as it is often referred to, is a "bare as you dare" event: some participants cover up, but others ride completely nude -- well, except for their shoes, which are recommended for safety reasons. And not everyone rides a bike; there are plenty of people who choose to ride along on skateboards, scooters or inline skates, or simply walk or jog.

This year's six-mile route -- organizers change it each year -- took the participants past famous Philly landmarks such as the Philadelphia Museum of Art, the Museum of the American Revolution, the Pennsylvania Convention Center, and the Cathedral Basilica of Saints Peter and Paul.

The purpose of the event is to promote the safety concerns of bicyclists and pedestrians as well as body positivity, and recognize the human dependence on oil.

"We advocate for cycling to be a major form of transportation," Maria Serrahima, the lead organizer of the Philly ride, told USA Today. "Advocating for people to feel comfortable on a bike is advocating for the Earth and sustainability and for conscious fuel consumption."

Similar rides have taken across more than 70 cities the U.S. and 20 countries, according to WorldNakedBikeRide.org, as a peaceful way to protest.

And while the event is not legal, Philadelphia officials give it a pass since this is "a nonviolent, infrequent event."

Lori M. Nichols may be reached at lnichols@njadvancemedia.com. Follow her on Twitter @photoglori. Find NJ.com on Facebook.