The National Park Service said it has cited 20 people for violating a closure order at Valley Forge National Historical Park.
The park in Montgomery County is closed because of the federal government shutdown.
The Park Service said runners and walkers have gotten only warnings, while those cited drove past barricades or blocked entrances.
Marathon runner John Bell of Chadds Ford said he did neither when he parked his car Sunday in a lot on Route 23, which bisects the park and remains open. He plans to challenge his $100 ticket.
Bell said he knew the main parking lot was closed, but didn't know people couldn't run in the park.
Safety is a concern given staff furloughs, especially after a hiker was injured last weekend at Acadia National Park in Maine, said Michael Litterst, a spokesman for the National Park Service.