The explosion happened at Old World German restaurant on Center Avenue in Huntington Beach around 8:30 p.m. Saturday. The restaurant was holding an Oktoberfest event at the time.
Huntington Beach Fire Department officials say there were four injuries - two firefighters and two civilians. The injuries were not considered life-threatening.
Witnesses say they saw a series of three explosions only about a minute apart. The firefighters were injured when they were investigating the initial explosion and another one went off.
The explosions triggered chaos and sent hundreds of people fleeing for safety.
"All of a sudden this giant explosion happened at the entrance," said witness Nicole Hallas. "We could feel the heat of the flames on our foreheads. At that point we just saw everyone running and exiting the whole event."
In the initial chaos multiple ambulances were called to the scene as firefighters had initially thought as many as 30 had been injured, but they revised that number after obtaining more information.
Fire officials say an underground electrical vault exploded, but the cause remains under investigation.
Huntington Beach Fire Department Battalion Chief Jeff Lopez said it was fortunate the injuries were not more serious, given the force and heat of the explosion.
"Electrical fires, vault explosions - a lot of energy potential, a lot of heat," Lopez said. "With the reports I'm getting on the level of injuries we have, it's not that bad."