Malfunctioning fireworks are being blamed for a blast that injured 28 people at a 4th of July celebration northwest of Los Angeles.

Around 20 people had to be taken to hospital late on Thursday night with minor to serious injuries after the explosion at the Simi Valley park in California.

Videos posted online show a series of large unexpected blasts on or near the ground.

One man watching the scene is heard saying: “That wasn’t supposed to happen.”

Witnesses described how large rockets, that were meant to shoot and explode high in the sky, were flying horizontally into the crowd before detonating.

“We saw at least two or three fireworks go sideways, horizontally and explode,” said JT Alpaugh, a KABC-TV reporter who was watching the display with his family.

“And then it seemed like the barrage of fireworks started exploding at the base of the park, right near the soccer field at ground level. Now obviously fireworks are designed to go up 200ft in the air and explode. And we realised immediately that something was terribly, terribly wrong.”

Four of the injured spectators are said to have serious wounds consistent with shrapnel injuries.

“There was an accidental detonation of a professional fireworks show that resulted in the cancellation of the show,” Police Commander John Parks told the Ventura County Star.

The annual city-run Fireworks Extravaganza usually attracts thousands of Independence Day revellers.

The display field was evacuated and cordoned off. The Ventura County fire department’s bomb squad was called in to deactivate the remaining fireworks.

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