protester

Concise Oxford English Dictionary © 2008 Oxford University Press:
protest

▶noun /ˈprəʊtɛst/ 

• 1 a statement or action expressing disapproval of or objection to something.

  ■ an organized public demonstration objecting to an official policy or course of action.

• 2 Law a written declaration, typically by a notary public, that a bill has been presented and payment or acceptance refused.

▶verb /prəˈtɛst/ 

• 1 express an objection to an action or remark.

  ■ engage in public protest.

  ■ N. Amer. publicly protest against.

• 2 make an emphatic declaration or denial in response to doubt or accusation.

• 3 Law write or obtain a protest in regard to (a bill).

– derivatives
protester (also protestor) noun,
protesting adjective,
protestingly adverb.

– origin ME (in the sense ‘make a solemn declaration’): from OFr. protester, from L. protestari ‘assert formally’, based on testis ‘a witness’.