Protester

[Hina Muqadas]

One who protests; one who utters a solemn declaration.

[Zipporah Schumaker]

A person who seriously engages in [the hobby] of protesting. They [run around] screaming trying to change the world by getting them to do the right thing instead of finishing college, [getting a job], or running for office.

[Dave Paprocki]

A [professional] [tester].

[Julene]

someone who shows that they disagree with something by standing somewhere, shouting, carrying signs, etc.