About 20,000 people came to the event, said David Hicks, a spokesman for San Diego's Regional Planning Agency.

The 3.5-mile portion of State Route 52 completes the 17.7-mile link between state Route 67 in Santee and Interstate 5 near La Jolla. Its completion is expected to remove heavy traffic from Mission Gorge Road and Mast Boulevard

The first segment of freeway opened in the 1970s in Rose Canyon. Construction on the road's final link began in 2008 and cost about $520 million, paid for with a half-cent sales tax and state and local funds, Hicks said.

The grand opening of the final segment of the Route 52 freeway began with a ribbon cutting ceremony at 9 a.m. near Cuyamaca Street, Hicks said.

The celebration continued with a 3-mile bike ride-run-walk on the new road.

The first 3,500 who arrived at the event got free "SR 52 Sea to Santee" T-shirts, and children took trackless train rides on the new road's lanes, Hicks said.

Aircraft from the Gillespie Air Show performed flyovers for the celebration, and guests also took in a car show, wandering clowns, balloon sculptors, gymnastics demonstrations and high school bands.

At the celebration, free T-shirts and food were handed out and bands played as bicyclists rode on the soon-to-be busy freeway lanes.

On a more serious note, the San Diego County Sheriff's Department brought its car crash display.