EXCLUSIVE

Justin Bieber came face-to-face with a group of NYPD officers Friday night ... and even though they took a picture of his mug -- it was only out of appreciation for his generous donation.

Bieber was eating at the Comfort Diner in NYC when he noticed a few officers eating next to him ... so when it came time to pay up, Justin coughed up the dough for their tab -- around $200.