The 25-year-old female deputy had responded to a call of a person acting erratically in the 8500 block of Alondra Boulevard around 5:40 p.m.
Before she could even get out of her car, the suspect started attacking her, punching her through the window of the cruiser, according to Sheriff Alex Villanueva.
She got out of the car and gave chase to the suspect on foot through a nearby parking lot. As the foot chase crossed Alondra and Downey Avenue, she was struck by a Chevrolet Suburban heading eastbound.
The deputy from the Lakewood station sustained major injuries and was transported to a hospital in Lynwood.
Other deputies who had responded to the scene took the original suspect into custody and booked him for assault with a deadly weapon against a peace officer.
Villanueva says the driver of the Suburban was driving under the influence and was arrested for felony DUI.