Three residents suffered serious injuries in the blast at 6 p.m., the fire department said, NBC News reported.
Nine Los Angeles Police Department officers, one federal Alcohol, Tobacco, Firearms and Explosives agent, and three other people sustained minor injuries, fire officials said.
The LAPD tweeted that bomb squad officers were seizing more than 5,000 pounds of illegal fireworks. Some of the fireworks were in the process of being detonated when the blast happened, multiple officials familiar with the incident told NBC News.
"Some of the fireworks were being stored in our Bomb Squad trailer as a precautionary measure," the LAPD tweeted. "Unknown at this time what caused an explosion."
The blast damaged nearby businesses, structures and cars, officials said.
News helicopter footage showed what appeared to a bomb squad disposal unit with heavy damage in the Historic South-Central neighborhood of Los Angeles, south of downtown. The windows of a laundromat were shattered.
The Los Angeles Fire Department said several structures sustained varying levels of damage. Search and rescue members were evaluating the structural integrity of the most heavily damaged building.
ZZ/PR