Roughly a dozen dirt bike riders on Wednesday snarled traffic on southbound Highway 101 in San Francisco, before some were caught on camera punching and kicking a Toyota Camry driver, who, according to a witness, tried to drive around them, but instead ended up with a broken leg.

The California Highway Patrol is investigating the altercation that sparked around 6:15 p.m. near the Cesar Chavez exit, and was partially captured on cellphone video. A group of between 10 to 15 dirt bike and ATV riders were slowing down traffic when the driver of a white Toyota Camry tried to maneuver around them, Officer Vu Williams said.

According to a San Francisco resident, who witnessed the exchange but asked to remain anonymous, one of the dirt bike riders pulled up beside the driver and swerved toward him. The bike appears to fishtail, but the rider righted himself, said the witness, who recorded the skirmish, which he deemed "really odd."

A red car, which the witness said may have been associated with the dirt bike riders, also slowed in front of the Camry, effectively boxing him in. The driver pulled over to the side of the freeway, where he was surrounded by the remaining bike riders.

The ATVs and dirt bikes did not have license plates on them, according to Williams.

Investigators say gangs of dirt bike riders have been reported causing issues in various parts of the city for years. CHP officers say catching them has been tough because the bikes can go off road into hiding in seconds.

"They get on the road, doing their stunts, and then they flee," the CHP said.

Police warn drivers not to face off with the bikers, and suggest backing off and calling police.

"The level of what they are doing, the level of danger, how brazen they've become has risen exponentially because they continue to get away with this," CHP Officer Vu Williams said.

The witness Wednesday said he was unable to pinpoint whether the driver was dragged out of his car or if he disembarked of his own volition.

The bikers got into a verbal altercation with the driver, Williams said, adding that they struck the car with their bikes and fists. The witness noted that the riders smashed the vehicle's side windows and damaged its panels.

The man was also beaten and kicked, and then dumped in the middle of the freeway, while the bikers fled the scene, Williams said.

A gang of bike riders were caught on camera attacking a driver and vandalizing his car on Highway 101 in San Francisco.

The witness was among a group of drivers who stopped to help the injured man. Not only was one of his legs broken, but his face had also been bloodied and he had road rash on his arms. Although the victim was coherent, the witness, who called 911, said he was "really in agony."

A tow truck driver who had some experience administering first aid helped the man, who was then loaded into an ambulance and rushed to a nearby hospital.

The witness, who moved to San Francisco from New York, recalled a 2013 incident during which a motorcycle gang assaulted a man who was driving a Range Rover with his wife and son on the Henry Hudson Parkway.

When he saw Wednesday's clash, the man immediately wanted to stop his car and come to the victim's aid, but he felt "helpless" because he was outnumbered. That's why he decided to record the incident in the hopes that it would help authorities identify, arrest and prosecute the suspects.

"They're doing wheelies on the highway, they're trying to control the flow of traffic," the witness said. "This is not the way that traffic should operate ... They are a liability to others on the road."

Anyone who may have witnessed the incident should contact the CHP office at 415-557-1094 between 8 a.m. and 5 p.m. Monday through Friday and ask for a watch officer. Outside of these hours, witnesses can contact the CHP Golden Gate Communications Center at 415-551-4100.

NBC Bay Area's Rick Boone and Kim Yonenaka contributed to this report.