The attack happened Tuesday at a popular off-leash park in San Francisco's Dog Patch neighborhood.
"And the next thing I knew, he had her pinned on her back," dog owner Ray Kang said.
Kang says he was taking his dog, Skyler, for a walk through the park when the 10-year-old beagle and labrador mix was attacked.
"Within seconds she was in the pit bull's jaws and the pit bull was shaking her like a rag doll," Kang said.
Photos reveal injuries Skylar sustained after being mauled. The pictures show deep punctures through Skylar's back and a major tear on her left side that required multiple stitches.
"I questioned why, you know, he wasn't in control of his dog in the first place," he said.
Kang says he confronted the man he thought was the owner of the pit bull.
"They just started walking off, they were fleeing the scene," Kang said.
A witness captured the exchange with a smartphone. Kang filed a police report just hours after the attack.
Officials told ABC7 News the incident is under investigation and that they're working to identify the pit bull and its owner.
"I've been coming here 25 years," dog owner Carma Keats said.
Those who use the park regularly are worried that the negative publicity will give ammunition to opponents of the off-leash park.
"If one child is a bully or acts out in school, it ought not reflect on the entire student body," Keats said.