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A Customs and Border Protection agent repeatedly struck a man who was lying on the ground handcuffed ... with the violent incident caught on video.

The brutal beating took place near Evanston, Illinois -- a suburb of Chicago -- Friday ... with the video starting off after an agent had already wrestled a man to the ground and slapped a pair of cuffs on him.

The agent brings his fist down on the detainee's face multiple times in the short clip ... bringing forth a chorus of screams from onlookers.

He then grabs the detainee by the back of the neck and forces his face into the ground ... though he ultimately lets up and just holds the man by the wrists before he and his fellow officer pull him to his feet and take him into custody. Getty

We reached out for comment ... and, a Dep

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