Staff at the Old World German restaurant on Center Avenue noticed an odor coming from the underground electrical vault. The fire department was called to the scene - where the restaurant was holding an Oktoberfest event - and the first explosion erupted shortly after, around 8:30 p.m.
Two firefighters were injured while investigating the initial explosion just before another one went off.
Witnesses say the lights started to flicker and the power went out before up to three explosions only about a minute apart rocked the restaurant.
Huntington Beach Fire Department officials say two firefighters and two civilians were injured, including a restaurant employee who led fire crews to the vault. He suffered burns to more than 38% of his body and was listed in moderate to critical condition Sunday.
"It's not abnormal that in an underground vault, when there's one explosion, it can be followed by others. In this case, there was, so perhaps there were three," said Jeff Lopez with the Huntington Beach Police Department.
The explosions, which reportedly sent flames shooting up dozens of feet into the sky, sparked chaos and sent hundreds of people fleeing for safety.
"All of a sudden, this giant explosion happened at the entrance. We were walking towards it, and then we could feel the heat of the flame like on our foreheads," witness Nicole Hallas said. "At that point, we 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. They later revised that number after obtaining more information.
Fire officials say an underground electrical vault exploded, but additional details on the cause have yet to be released.
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."
Southern California Edison crews worked overnight to replace the transformer and begin their investigation.
"The first priority in these situations is to get the damage cleaned up, and make sure we repair and restore power as quickly as possible," said Robert Villegas with Edison.