During a May Day protest in downtown Los Angeles, a protester was caught on video (below) sucker punching a female LAPD officer from behind.
The unidentified man, who police are hoping to identify, slammed a metallic object on top of the officer's head.
LAPD Chief Charlie Beck said: "This is a serious crime against a Los Angeles police officer who was just doing her job."
Chief Beck added that the female officer had a minor concussion, but was resting at home. He is calling for witnesses to come forward to identify the attacker, whom he described as "cowardly."
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Occupy protesters and activist groups held a rally on Monday for May Day, which began in Chicago decades ago, and is seen as an important moment in the labor movement.