Rioters

Rioters serve as minor antagonists In Batman: Arkham Knight. They are also serve as the main antagonists in The Line of Duty.

Arkham Knight Incident
Several rioters could be seen throughout Gotham City causing riots. Batman saved one of the GCPD Officers, who was being held hostage. After Firefly caused the firetruck carrying the crew of Station 17 to crash, the rioters took this opportunity to capture the firefighters, including Raymond Underhill. When Scarecrow activated the Cloudburst, and released his Fear Toxin into Gotham, most of the rioters were affected, while some managed to escape into buildings or high ground. When Poison Ivy saved Gotham, rioters continued robbing Gotham, although many had died from Scarecrow's attack.

Knightfall Protocol
After Batman arrests all of Gotham's Most Wanted, rioters are still seen in Gotham. They were presumably stopped by the GCPD afterwards.

Trivia

 * Founders' Island is the only island in which riots do not happen.
 * During the main story, a rioter might occasionally take the appearance of the Joker.
 * When investigating unconscious rioters after a fight, the thugs have a damaged appearance, with bruises and occasional cuts being the most noticeable.