The truck driver, who was alone in the vehicle, struck a guardrail and fell 60 feet onto a vendor's booth at Chicano Park, California Highway Patrol Officer Jake Sanchez said. He was taken to the hospital with major injuries and arrested on suspicion of driving under the influence causing deaths and injuries, the officer said.

Authorities identified him as Richard Anthony Sepolio (25), a US Navy member stationed at the naval base on Coronado Island across the bay from San Diego.

"It's horrible. It's horrific. We had innocent people down here having a good time and now they're gone," Sanchez said.

Eight people on the ground were injured. One suffered major trauma and seven others had minor to moderate injuries, said Lee Swanson, a spokesman for the city's Fire-Rescue Department.

Witnesses said the GMC pickup truck with Texas license plates landed on a vendor's booth set up for La Raza Run, a motorcycle ride that ends with a celebration at the park and typically draws hundreds of participants. The crash steps away from a stage where a rockabilly band was playing.

"I saw a truck come right off the freeway. It was going so fast it flew over the stage and landed in front of the stage on a tent, a booth that was set up," Chase Dameron told The San Diego Union-Tribute.

He said four people in the booth were crushed by the truck.

"It was like a movie. It was like in slow motion," Dameron said. "Where it hit, there was dust and debris and instant chaos and panic. People running crazy."

Another witness said the truck hit the front end, bounced and landed on its side.

Photos from the scene show the truck's front end was crumpled and its hood popped open.

The park is located beneath the bridge in a predominantly Hispanic neighbourhood in central San Diego.