Blake Griffin -- ALLEGEDLY BITCH-SLAPPED CLUBGOER ... Cops Investigating

Blake Griffin has been accused of bitch-slapping a 39-year-old man inside a nightclub in Vegas this weekend ... TMZ Sports has learned.

Here's what we know ... a man named Daniel says he was partying at Tao nightclub in the Venetian in the early hours of October 19th when he tried to take a cell phone pic of the crowd.

According to a statement Daniel gave to Tao security, the man says Blake (who was partying at the club with his L.A. Clippers teammates) was bothered by the flash of the camera -- and "grabbed my phone with one hand."

Daniel says he didn't let go of the phone because his ID and credit card were attached to it -- so Blake grabbed his neck and began to squeeze while demanding Daniel give up his phone "or else."

According to the statement, the two eventually separated after Blake took the phone -- and when Daniel tried to ask for it back, Blake struck again ... grabbing his neck and squeezing for a second time.

Daniel says when security finally got to the scene -- Blake told them, "Get him the f**k out of this club."

Daniel says Blake then "slapped me in the right side of my face."

After he left the club, Daniel filed a battery report with the Las Vegas Metro Police Department. He says cops came to his hotel the next day and spoke with him for about an hour.

So far, attempts to reach Blake for comment have been unsuccessful.

Story developing ...