Four people are dead and nine injured after an out-of-control pickup truck plunged off the San Diego-Coronado Bridge and nose-dived into a crowd gathered at a festival below.

The truck driver, who was alone in the vehicle, struck a guardrail and fell 18 metre onto a vendor’s booth at Chicano Park, California Highway Patrol Officer Jake Sanchez said on Saturday.

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 licence 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 occurred in front of 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-Tribune.

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 bonnet popped open.

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