The Celtics vs. Knicks action wasn't just limited to inside TD Garden on Wednesday night ... some of their fans got into a wild brawl outside the venue as well -- and a good portion of the scrap was captured on video.
Check it out ... just after New York stunned Boston with another come-from-behind road win, several people clad in blue and orange mixed it up with some C's backers.
A man in a Paul Pierce jersey appeared to get the better of one Knick fan -- bloodying him with a flurry of punches.