the.com/demonstrator

means Someone who takes a cause out of their head and into the street, showing their objection in public rather than just stating it.

from From Latin demonstrare — de- ('completely') plus monstrare ('to show, to point out'), the same root that gives us 'monster' (literally, a thing shown or pointed at as an omen). A demonstrator, then, is etymologically just someone pointing very hard at something — the march is the finger, the crowd is the gesture.