Boston Celtics Fans Get Into Brawl At NBA Finals Watch Party

The Boston Celtics and Dallas Mavericks weren't the only ones going head to head in the NBA finals ... a bunch of Celtics fans got into an amateur boxing match at a game 4 watch party.

Friday night's brawl broke out between two men seated near each other in the crowd at TD Garden arena in Beantown ... where Celtics fans were having a viewing party of the on-court action taking place inside American Airlines Stadium in Dallas.

Check out the vid ... the younger dude was standing over the older guy and using his head as a punching bag. Several bystanders and a security guard tried to pull the aggressor off the old man as he cowered on cement stairs.

For some reason, the older guy stood up, climbed the stairs and started sparring with another man. All three fighters were wearing green Celtics jerseys.

It's unclear if anyone was ejected from the stadium or if police were called in to investigate. We've reached out to the Boston PD for comment.