When officers arrived at the Vons supermarket they found the customer outside suffering from multiple stab wounds.
The victim was taken to hospital, where he was listed in stable condition, but refused to identify himself or give investigators any details about the incident.
Officials say there were multiple witnesses and that the security guard has “fully cooperated” with the investigation.
Authorities say the incident unfolded when the customer went into the store without a mask and “started to cause a scene.”
The man was then approached by store employees and allegedly became “belligerent” and assaulted a member of staff.
A physical altercation developed when the security guard intervened and tried to escort the man out of the store, police say.
“At one point during the fight, the security officer pulled out a small pocket-knife and stabbed the customer multiple times,” said SMPD.
The security guard was the person who called 911 and told officers he had acted in self-defence.
No arrests have yet been made following the incident.
Los Angeles County has a mask mandate in place for public indoor settings and a sign outside the store notified customers they were required to wear one.
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