Fire spokesman Matt Spence says the cause of the Wednesday evening explosion was not immediately clear.
He says paramedics took four people to hospitals. Two of them were in critical condition and two in fair.
The blast occurred at about 6:30 p.m. inside Bemis Meat Market, a small grocery store in a largely vacant industrial zone about a mile northeast of the University of Southern California campus, near Interstate 110.
There were no visible signs of the explosion from outside the market, with only a boarded-up hamburger stand nearby.
Police tape cordoned off an area of about two blocks around the market.
