A 100-year-old man backed his car on to a pavement and hit 11 people, including nine children, opposite a South Los Angeles primary school just after classes had ended.
Four of the children were critical when firefighters arrived but they were stabilised and now in a serious condition at a hospital, city fire captain Jaime Moore said.
The blue Cadillac backed slowly into the group of parents and children buying snacks from a pavement vendor, and the crowd banged on his windows and screamed for him to stop, but not before some of the children were trapped under the car, witnesses said.
Children's backpacks, shoes, sweets and loose change were strewn around the scene behind a discount grocery store opposite the school, Main Street Elementary.
Police identified the driver as Preston Carter and said he was being co-operative. Mr Carter talked to reporters after the crash, five miles south west of central Los Angeles. He said he had a driving licence and will be 101 on September 5. "My brakes failed. It was out of control," Mr Carter told KCAL-TV.
Asked about hitting the children, he said: "You know I'm sorry about that. I wouldn't do that for nothing on earth. My sympathies for them."
Mr Carter was pulling out of the grocery store car park, but instead of backing into the street, he backed on to the pavement, police captain George Rodriguez said. "I think it was a miscalculation on his part. The gentleman is elderly," he said. "Obviously he is going to have some impairment on his decision making."
Older drivers have been involved in other tragedies. In 2003 an 86-year-old man mistakenly stepped on the accelerator of his car instead of the brake then panicked, ploughing into an open-air market in Santa Monica. Ten people were killed and 63 injured.
According to California's Department of Motor Vehicles, people over 70 must renew their driving licence in person, rather than via the internet or by post. Older drivers can also be required to take a supplemental driving test if they fail a vision exam, or if a police officer,doctor, or a family member raises questions about their ability to drive.
Capt Rodriguez said the collision was being investigated as an accident and Mr Carter was not under arrest.