Even before a ribbon-cutting to unveil newly installed sculptures at Paine's Park on Thursday afternoon, the geometric forms were heavily scarred with scratches and tire marks left by skateboarders and BMX bikers who had attacked it overnight.

But unlike other public works popular with skaters (see the Claes Oldenburg paint "blob" on Broad Street that was reoriented a few years ago to deter them), this was exactly the intended result.

The piece, Steps and Pyramid, is British artist Jonathan Monk's skatable reinterpretation of sculptures by the minimalist artist Sol LeWitt. It's also the first work to be completed as part of Open Source - a series of art installations and events that the Mural Arts Program is calling the largest site-specific public art festival in Philadelphia's history.

Thirteen more installations, murals, and sculptures (and one artistically motivated waste-transfer station) will be unveiled through the summer, culminating in a citywide festival in October.

"This exhibition offers a bold redefinition of contemporary art, and it will uplift Philadelphia as a culturally innovative global city," said Mural Arts executive director Jane Golden. (She confessed to being a former skate rat herself, having taken up the pastime while working on the 1970s movie Skateboard, and giving it up at age 30 when her skateboard was run over by a bus.) "It will be unlike anything done before, and we expect people to come here from all over the world."

The works - by internationally known names including Shepard Fairey, JR, and MOMO, as well as Philadelphia-based artists such as the Dufala Brothers, Amber Art and Design, and Odili Donald Odita - tap into Mural Arts' tradition of collaborating with community members, including students, immigrant groups, inmates at Graterford Prison, and residents of a halfway house.

Works will range from the figurative, as in portraits of people dealing with addiction to be placed around the city by Caledonia Curry, known as Swoon, to the conceptual, like a 2,500-square-foot maze that Sam Durant plants to install on the Municipal Services Plaza, in a reference to the labyrinthine criminal justice system.

Pedro Alonzo, a Boston-based independent curator, selected the 14 participating artists. He said Monk's work was developed in conversation with a group of eight teenage skaters.

"It reflects concerns about the passive process of looking at art," he said.

Josh Nims, a skateboarder who was instrumental in opening the park, just south of the Philadelphia Museum of Art on the Schuylkill's bank, stood surveying the boxy forms interrupting the park's curving surfaces. He has been hoping to work with the museum to bring in skatable sculpture for some time, so for him this was the fulfillment of a long-held ambition.

But, he conceded, reviews of the works, at least from a skating perspective, were still out.

One skater after another shot up onto a ledge of the sculpture, then lost control and wiped out.

Finally, one landed gracefully, and Nims smiled. "The way it looks right now," he said, "I have a feeling it's going to be a success."