Three people were injured in the collapse, which occurred in a two-story fourplex in Harvard Heights.
The front of the apartment house at 1625 S. Westmoreland Ave., near Washington Boulevard, came down just before 9 p.m. Sunday, said d'Lisa Davies of the Los Angeles Fire Department.
"It was a 35 foot by 20 foot section that collapsed," Davies said. "At 9:30 p.m. we received notice that all occupants were accounted for."
The Red Cross offered to find temporary housing for the displaced families, but they declined and will be staying with family members, she said.
The DWP and The Gas Co. shut down power and gas, and after an inspection, the city's Building & Safety Department condemned the building, Davies said.
Three people trapped in the rubble were rescued, including a 36-year-old woman who had been on the second floor and suffered cuts to her head. She was transported to a nearby hospital, but is expected to be OK, Davies said.
Authorities have evacuated a neighboring eight-unit building, as the condemned structure is leaning toward it, Davies said.
Those residents were expected to return as soon as the damaged structure is stabilized, she said.
Some 70 firefighters, ambulance and search and rescue crews rushed to the scene when the front of the building collapsed, but everyone inside has been accounted for.
Neighbors and residents told Fox11 they heard a hissing sound like a natural gas leak and then a loud explosion, followed by chaos and panic.
Families with young children lived there, according to the TV station.
The families will be let back in today to get their belongings, and then the structure will be razed, authorities said.
The cause of the collapse is under investigation.