The explosion occurred around 12 p.m. at a textile business located in a row of commercial structures near 7th and S. Kohler streets, according to the Los Angeles Fire Department.
Three men were hurt, including two who were severely burned and in critical condition, fire officials said. The third patient was in fair condition after suffering unspecified injuries.
Witnesses reported the explosion was very loud and rocked the area.
The Fire Department declared a "major emergency" after 137 firefighters responded to the scene and were in a "fully defensive mode."
Just over an hour after the blast, the LAFD announced that the fire was extinguished. No other buildings were seriously damaged.
The cause of the incident is under investigation.