Agitator - a person who urges others to protest or rebel.
Activist - a practitioner of given action. An activist is someone who takes action in support of or opposition to a cause.
Activism can take a range of forms, from writing letters to government representatives to organizing boycotts. Some activists engage in radical or even illegal activity to further their ends, while others prefer to stay within the boundaries of the law to win more supporters to their causes. Every time someone writes a letter to the authority, educates a friend about an issue, or phones an elected official, he or she is participating in activism.