Document via video, take a fat lip, and then sue them and their union for all you can get. Just stay civil while it happens. One of those derelict robots will pop off, and if any part of their trip to the demonstration was paid by the fat cat unions, you have them by the short hairs. They were paid agents.
I had planned on telling that person “evil prospers when good men do nothing,” but I like YOUR post WAAAAY better ;-) I love how you think!