The Collaborative International Dictionary of English v.0.48:

Rioter \Ri"ot*er\, n.
   1. One who riots; a reveler; a roisterer. --Chaucer.
      [1913 Webster]

   2. (Law) One who engages in a riot. See Riot, n., 3.
      [1913 Webster]

WordNet (r) 3.0 (2006):

rioter
    n 1: troublemaker who participates in a violent disturbance of
         the peace; someone who rises up against the constituted
         authority

Moby Thesaurus II by Grady Ward, 1.0:

73 Moby Thesaurus words for "rioter":
   agitator, battler, belligerent, belted knight, bickerer, blade,
   bravo, brawler, bully, bullyboy, combatant, competitor, contender,
   contestant, disputant, duelist, enforcer, extremist, fencer,
   feuder, fighter, fighting cock, foilsman, frondeur, gamecock,
   gladiator, goon, gorilla, hatchet man, hood, hoodlum, hooligan,
   insubordinate, insurgent, insurrectionary, insurrectionist,
   insurrecto, jouster, knight, malcontent, maverick, militant,
   mutineer, nonconformist, plug-ugly, quarreler, rebel, revolter,
   revolutionary, revolutionist, rival, rough, rowdy, ruffian,
   sabreur, scrapper, scuffler, squabbler, strong arm, strong-arm man,
   strong-armer, struggler, subversive, swashbuckler, sword,
   swordplayer, swordsman, thug, tilter, tough, traitor, tussler,
   wrangler