The incident was reported shortly after 10:30 p.m. at a Vons in the 700 block of Broadway. The security guard told the police dispatcher that "he got into a fight with a customer and stabbed him in self-defense," the Santa Monica Police Department said in a statement.
"It was a tussle, and then the security guard got really upset and pulled out a pocket knife and stabbed the guy, and a lot of blood spurted everywhere," witness Andre Berryman said. "A lot of people came to break it up, pull them apart. But then that guy was bleeding profusely."
Officers arrived to find the customer in front of the store, suffering from multiple stab wounds, the news release said. The injured man was transported to a hospital, where he was listed in stable condition.
At the medical center, he refused to identify himself or provide any statement about the incident, authorities said.
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Officers interviewed multiple witnesses and the security guard, who "fully cooperated" with the investigation, the police statement said.
According to authorities, the incident began when the customer entered the store and without a mask and "started cause a scene." When he was approached by employees, he became "belligerent" and assaulted a member of the store's staff, police said.
A physical altercation ensued when the security guard intervened and attempted to escort the man out of the location, the news release said.
"At one point during the fight, the security officer pulled out a small pocket-knife and stabbed the customer multiple times," the Police Department said. The wounded man then exited the store, where he was found by responding officers.
Berryman said he believed the security guard was simply doing his job, adding that the man had been dismissive of the security guard before the altercation.
No arrests were immediately announced.
Los Angeles County has a mask mandate in place for public indoor settings. Signs outside the Vons notify customers that masks are required before entering the store.
Anyone with additional information about the altercation is to call the Santa Monica Police Department's Criminal Investigations Division at (310) 458-8451 or the watch commander at (310) 458-8427.
Correction: An earlier version of this article erroneously reported the day on which the incident occurred. The story has been updated.