It happened about 2:10 p.m. at 815 N. Sepulveda Blvd., according to Manhattan Beach police.
The victim was at the counter, about to order food when the suspect, mumbling unintelligibly, approached from behind, walked away and then returned, police said.
The suspect confronted the victim, who noticed the suspect was armed with a folding knife. When the suspect lunged toward the victim's stomach, the victim tossed his beverage at the attacker, raised his hands in defense and received a cut on his arm, according to police.
The attacker fled but was located nearby, identified by witnesses and arrested on suspicion of assault with a deadly weapon, police said. A knife was also recovered.
The suspect's name and age and were not disclosed and it was unclear if the victim was a juvenile or adult.
The victim received first aid from responding officers before he was taken by firefighters to a hospital, police said.
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