An off-duty cop was arrested today for assaulting a customer at a Hamilton Heights grocery store where he was moonlighting as a security guard, authorities said.

Paule Rivera, 35, was working at the Fairway Supermarket on 12th Avenue and West 132nd Street at 12:50 a.m. when he got into a fight with a customer, police sources said.

The fight escalated and turned physical and Rivera was arrested, police sources said.

Rivera was charged with assault, police said.