Donald Trump: Fists Fly ... Chicago Rally Cancelled

Tension at a Donald Trump rally finally reached boiling point ... as multiple fist fights broke out in Chicago forcing the candidate to pull the plug over safety concerns.

The crowd was fired up and cameras were rolling even before Trump arrived, and the video is intense -- supporters and protesters squared off all over the arena on campus at the University of Illinois at Chicago.

Since Trump wasn't there, microphones weren't on yet ... but the rage is still clear. This all started simmering much earlier in the day, outside the arena.

Worth nothing, this morning Trump applauded the North Carolina man who sucker-punched a protester.