Wendy Wu, a waitress at the Chinese restaurant, was in a walk-in freezer when the truck hit and wasn't hurt.
"There was a loud boom and a lot of shaking. I thought it was an earthquake," she said. Walking out of the freezer, she saw a refrigerator pushed across the room and furniture in disarray.
Speaking through an interpreter, Wu said she saw several injured people bleeding and trying 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.
Most of the 15 people hurt had minor injuries but one person — not a firefighter — was hospitalized in critical condition, authorities said.
The firetruck's front half remained embedded in the restaurant for hours, with shattered glass and rubble heaped on the sidewalk. Chairs and tables were scattered inside.
The truck finally was pulled free late Wednesday night.
The California Highway Patrol was investigating the crash between the Monterey Park engine and a ladder truck from neighboring Alhambra.
Both were heading to a house fire and had their lights and sirens going when they collided, CHP and fire officials said.
A Monterey Park firefighter received moderate injuries while two other city firefighters and three from Alhambra had minor injuries, fire officials said.
It was not immediately clear how fast the trucks were going but Monterey Park Fire Chief Jim Birrell said engines responding to emergencies typically "slow at an intersection and proceed when it's safe."