Most security guards aren't much anyway. Usually they're the guys with too much phony machismo to take a desk job, but not enough strength to take blue-collar-type jobs that require heavy lifting.
Not that that's always the case--I've seen some SG's who could hold their own. But if the people at the rally stood up to them, the law of averages would probably work in their favor.
You've got the right idea.
In Eugene, Oregon, the "conservation" mafia and anarchists (the children in black) get all the free legal help they need from the University of Oregon law school.