Source : Webster's Revised Unabridged Dictionary (1913)

Protester \Pro*test"er\, n.
   1. One who protests; one who utters a solemn declaration.
      --Shak.

   2. (Law) One who protests a bill of exchange, or note.

Source : WordNet®

protester
     n 1: a person who dissents from some established policy [syn: {dissenter},
           {dissident}, {objector}, {contestant}]
     2: someone who participates in a public display of group
        feeling [syn: {demonstrator}]