Police say the two men and two women were among a large crowd which had gathered for a motorcycle festival in a park below the Coronado bridge in San Diego.

At least eight other people on the ground were injured.

The truck driver lost control as he drove onto the bridge, hitting a guardrail before crashing to the ground.

It is thought those who died were in a stall selling T-shirts.

"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," said Chase Dameron, who was in the park. 

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

The California Highway Patrol said the dead were a 62-year-old man and 50-year-old woman from Arizona, and a 59-year-old man and 49-year-old woman from a suburb east of Los Angeles.

The driver, 25-year-old Richard Anthony Sepolio, was seriously injured and has been arrested on suspicion of driving under the influence.

He was stationed at a naval base in Coronado Island, across the bay from San Diego.

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

The truck landed on a vendor tent set up for La Raza Run, a motorcycle ride that ends with a celebration at the park and normally attracts hundreds of people. 

It happened in front of a stage where a rockabilly band was playing.