Police said the incident happened about 8 a.m. The victim was walking her dog on Fuller Avenue, just south of the popular hiking spot, when witnesses said the suspect suddenly attacked.
"He grabbed her and sort of threw her to the ground and then tried to actually take off her pants," said witness Bryce Bonn.
Some Good Samaritans not only stopped the brazen attack, but held the suspect down until officers arrived.
The suspect, identified as 21-year-old Dejan Sebastian, was being held on $100,000 bail.
The victim declined medical attention at the scene.