I’m not sure you can arrest someone for pre-crime. Going to a protest that is expected to get out of hand isn’t a crime. Unless they specifically threatened to do a crime… 
I just finished watching The Trial of the Chicago 7.
Evidently they were charged under the Rap Brown law, which is part of the 1968 Civil Rights act. The act makes it a federal crime to use interstate or foreign commerce routes or facilities (such as by crossing state lines or through the mail, use of the Internet, or phone calls) to incite a riot, organize, promote or participate in a riot or to extend activities of a riot, or to aid and abet any person performing such activities.
It seems to me that hundred if not thousands of people could be charged with this. Or, like in the trial, the organizers. Only unlike Chicago, the police weren’t the cause of the riot this time.
