One of the victims was in critical condition, officials said.
The crash on a busy commercial strip in Monterey Park left one truck embedded in the restaurant, with shattered glass and rubble heaped on the sidewalk. Chairs and tables were scattered inside.
"There was a loud boom and a lot of shaking. I thought it was an earthquake," said Wendy Wu, a waitress who was in a walk-in freezer when the truck plowed through the front of the restaurant.
When she walked out of the freezer, uninjured, she saw a refrigerator pushed across the room and furniture in disarray. Wearing a uniform with the restaurant logo and speaking through an interpreter, she said she saw several injured people bleeding and attempting to stand.
Waitress Vivian Lu arrived for her shift moments after the truck rammed the building. She said she saw "a lot of people covered with blood," including a pedestrian with a mangled leg who had been knocked into the restaurant.
Monterey Park Fire Chief Jim Birrell said trucks from his city and neighboring Alhambra were responding to a house fire shortly after 3 p.m. when they slammed into each other and one truck plowed into the restaurant.
Rosa Saca, a spokeswoman for Los Angeles County-USC Medical Center, said nine victims were brought there, including the five firefighters. The critically injured person was among the hospital's patients but is not one of the firefighters, she said.
Erik Jiang, a spokesman for Garfield Medical Center in Monterey Park, said six victims from the crash were being treated at the hospital. None was in critical condition, he added.
An investigation was underway to determine what caused the crash.