The incident was reported shortly after 2:45 p.m. at the McDonalds at 2401 University Ave., according to Menlo Park Fire Department Chief Harold Shapelhouman.
Firefighters arriving on the scene found six people with gunshot wounds, four inside the restaurant and two outside, but it is unclear where they were shot, Schapelhouman said.
One person, an adult male in his 20s, received more serious injuries and was immediately taken by ambulance to Stanford Medical Center for treatment.
Five others, ranging from an apparently elderly woman to a child somewhere around age six, received less serious injuries, mostly to the lower extremities, Schapelhouman said.
All victims were taken to Stanford Medical Center for treatment.
East Palo Alto Police, including Chief Ronald Davis, are on the scene now, Schapelhouman said.
