By Marty Graham

SAN DIEGO (Reuters) - A U.S. Navy serviceman was under arrest on Sunday after a pickup truck he was driving while intoxicated hurtled off a ramp leading to San Diego's Coronado Bridge and killed four people at a motorcycle rally and festival in a park below the span, police said.

The four victims were in a vendor's booth at about 3:30 p.m. PDT when the truck plunged off the bridge's on-ramp and fell 60 feet (18 m) into Chicano Park, the California Highway Patrol and local media reported.

Stunned witnesses described seeing the truck flying from overhead and crashing into a crowd of hundreds of people who were gathered below.

The truck "was going so fast it flew over the stage and landed in front of the stage on a tent," Chase Dameron told the San Diego Union-Tribune.

"It was like a movie. It was like in slow motion," he added, describing the scene as "instant chaos and panic."

In addition to the four fatalities, nine people, including the driver, were injured but authorities did not disclose their conditions.

"They are innocent people that are just down here having a good time, and now they're gone," said CHP public information officer Jake Sanchez. "It's horrible. It's horrific."

Sanchez said the truck was traveling northbound on Interstate 5 when the driver lost control while exiting the freeway and crashed into a guardrail, sending the vehicle plummeting into the park below.

The driver, a Navy sailor named Richard Sepolio, according to local media, was charged with driving under the influence of alcohol, Sanchez said. He was being treated for serious injuries at a University of California at San Diego hospital.

The suspect is either 24 or 25 years old, Sanchez said, and assigned to the North Island Naval Air Station on Coronado Island. He did not disclose his rank or hometown. Navy officials were not immediately available for comment.

The air station, one of several large naval installations in the Southern California city, lies across San Diego Bay from the downtown. It is about four miles from the crash site.

The fatalities included a 62-year-old man and a 50-year-old woman from Chandler, Arizona; and a 59-year-old woman and 49-year-old man from Hacienda Heights in Los Angeles County, California, an NBC News-affiliate reported.

The California Highway Patrol was not immediately available on Sunday for further comment.

(Additional reporting by Curtis Skinner, Chris Michaud and Laila Kearney in New York; Editing by Alan Crosby)