So it was interesting to learn that the protesters got hold of a lot of bona fide tickets. There were hundreds of them inside the arena and thousands outside. Most of the insiders were playing mum, planning to disrupt the rally while Trump was speaking. However, some protesters (there was more than one organized group plus freelancers) were starting their protests early, and being hustled out. Among the Trump supporters (roughly 6000), there was a lot of suspicion. Perhaps security got wind of what was likely to happen, and cancelled the rally.
There were fist-fights after the announcement (purportedly--no pictures that I saw). If anyone was seriously hurt, it didn't make the news. Trump called the protesters thugs, but they weren't very thug-like if so few engaged in fights. The Trump supporters weren't too thuggish either.
The reason I looked into the details is because Jonathan Chait wrote a scare piece about how leftists who are against free speech squelched the rally. Nothing I read made me worry that the protesters wanted anything but to express their opinions in a very noticeable way. Most were even hoping to avoid battery, so I think Jonathan Chait should calm down.
Trump was probably right to cancel the rally. It would've been a big embarrassment for him, one way or another. Nonetheless, I hope to see more big counter-demonstrations against Trump. Maybe I'll even go to one.