SAN DIEGO -- An 18-year-old man was in custody Friday on suspicion of riding an off-road motorcycle onto a football field during a high school graduation in Lakeside, authorities said.

Chad Leonard allegedly resisted arrest during the incident Thursday evening at El Capitan High School, injuring a sheriff's deputy in the process, said Sgt. Damon Blankenbaker of the San Diego County Sheriff's Department.

Leonard and an unidentified companion, who remains at large, rode onto the field on separate off-road motorcycles around 6 p.m., Blankenbaker said.

The companion struck a 17-year-old bystander as he fled and faces hit-and-run charges if found, Blankenbaker said, adding that extra deputies were called in after a large crowd upset with ceremony-crashers began to gather.

The conditions of the injured deputy and struck bystander were not immediately released.