Man Slaps Woman on NYC Subway, Cops Investigating

A train ride in New York turned violent for one woman, who got a hand to the face after getting into an argument with an alleged creep ... a guy NYPD's now trying to find.

The incident went down on a Brooklyn-bound subway line early Friday ... and as you can see in the video ... the man and woman were really going at it. The woman was seated and yelling at the guy who was standing over her and shouting back, "It's none of your business."

At one point, things escalate physically, with the guy smacking her right in the face ... and she lets out a scream.

Law enforcement sources tell us ... someone on the train reported this to cops shortly after it happened, and the victim suffered minor injuries and a pair of broken glasses. The man in question had fled by the time officers arrived.

As to what led up to this, we're told the woman allegedly saw the guy taking photos of women onboard, and she decided to do the same to him ... which apparently pissed him off.