The robbery occurred at David's Jewelers in the 10900 block of Main Street. The business later released the video in hopes of helping police identify and arrest the hammer-wielding thieves.
The footage shows at least three suspects, each wearing a mask and a black hooded sweatshirt, smashing glass jewelry cases as employees throw an office chair and a trash can at them.
The violent confrontation continues in front of the building after the thieves ran outside, according to a separate surveillance video.
The suspects then fled the scene.
The business owner estimated that about $1 million worth of merchandise was stolen, a spokesperson for the El Monte Police Department said.
On social media, the jewelry store posted a photo of a black pickup truck that was being sought in connection with the crime.