"Peaceful" would be the operative, defining word in civilized society. If the demonstration is not peaceful, then it is not civilized and therefore not "free speech" but merely a riot of uncivilized animals. The conundrum is that denying a demonstration before it even takes place is based on a huge assumption: that one is dealing with uncivilized animals incapable of holding a peaceful protest/demonstration. What is sad is that evidence, behavior, and that violent, in past demonstrations proves this assumption correct.
And where does this lead? More protests, about not being able to protest, of course.