Correct spelling, explanation: protester derives from protestari, which is a word of Latin origin and used to mean to declare openly, which is quite similar to the contemporary meaning. Today, however, the word protester is not a verb, but a noun that refers to a person who declares something openly.

Definition of protester:
1. noun – a person who takes part in demonstrations and protests,
He was a very aggressive protester and the police decided to arrest him.

Expressions with protester:
The word protester often goes together with the following words: peaceful, angry, demand, political:
He is a peaceful protester who only wants to speak out his opinion.
The authorities didn’t know what to do with an aggressive protester.
The protester demands justice for the innocent victims of the crime.
He is a political protester with a very idealistic set of beliefs.

Incorrect spelling, explanation: the word protestor is often spelled as protester, and, interestingly, many sources claim that both forms are correct. However, it is not true as the form protestor used to be correct in the past, but now most scholars agree that the form is archaic and the only correct option is protester.