Technically, the constitutional right is "the right of the people peaceably to assemble". And the wikipedia summary is pretty good: "Unlawful assembly is a leg

Technically, the constitutional right is "the right of the people peaceably to assemble". And the wikipedia summary is pretty good: "Unlawful assembly is a legal term to describe a group of people with the mutual intent of deliberate disturbance of the peace. If the group are about to start the act of disturbance, it is termed a rout; if the disturbance is commenced, it is then termed a riot." So in other words, if the police allege you were there not to protest but to start a riot, they don't actually need to wait for the riot to start. You are probably thinking "man, it seems like the police can abuse that law just by always claiming the crowd looked like they were going to become violent". And you are correct, but that's the way it is.