"She stopped a total of about three times and the same subject was continuing to follow her," Palo Alto Police Det. Sgt. Brian Philip said.
Philip says the woman, in her 30s, was jogging around 10:30 a.m. when a man who was following her suddenly attacked her from behind and groped her.
"She was able to fortunately fight him off and two cyclists came to her aid as well and I think the culmination of all that scared the person away," Philip said.
Palo Alto police say it was actually the second of two incidents at the Baylands Nature Preserve. They believe the same man followed another woman 45 minutes earlier, but stopped short of physically attacking her.
The man is believed to be around 20 years old, 5 feet 8 inches tall, weighing 160 pounds, with black, buzzed hair. He was also wearing clear-colored studded earrings in both ears and had possible tattoos on both arms.
Regulars of the Baylands trail say they had always felt safe, considering its popularity and its proximity to the Palo Alto Golf Course. But now, many say they'll be making adjustments.
"Keep an eye on where other people are, you know, kind of watch out for each other," Margaret Cardiel said.
Palo Alto police plan release a composite sketch of the attacker Tuesday. Anyone with information pertaining to the incident may contact Palo Alto police's 24-hour dispatch center at (650)329-2413.
Bay City News contributed to this report