Shocking footage has emerged of a driver being attacked by a biker gang on a highway in San Francisco, USA.

The video, captured on Thursday on Highway 101, shows a dozen people riding dirt bikes and an all-terrain vehicle chasing a white Toyota Camry and forcing the driver to stop.

Then, they can be seen surrounding the vehicle while one of the gang beats the driver up.

"I have only lived in SF for 18 months, but this is the 3rd time I've seen this group of unlicensed dirtbikes riding around the city," the filmer wrote online.

"Me and two other vehicles stopped to help the man, laying on the middle of the offramp."

"He clearly had a broken leg, bloodied face, and road rash on his arms. He was in agony," he added.

"The tow truck driver had medical training and attended to the man, and I called 911."

"The man who was assaulted is a Lyft driver (according to the sticker on his windshield), and it appeared he was on duty, because he had an iPad on and mounted on his dash," he continued.

"He was the only person in the vehicle."

Reports say the driver was taken to hospital with a broken leg and other non-life-threatening injuries.