Riot is an Emergency event that can occur in your prison during normal game play.
The main cause for a riot to occur is when your Prisoners become very unhappy. If the Needs of a prisoner are not being met, then he will slowly or quickly become irritated depending on the need. If the need never gets satisfied, then the prisoner gets angry and will start causing problems. An angered prisoner who is in his cell will start destroying the Objects in it, but if he's outside his cell then he will start attacking other prisoners and Staff members. There are some needs which don't make prisoners angry if not satisfied, like Clothing and Comfort.
As more prisoners become angry in your prison, the higher the Danger level becomes. If and when that danger level gets too high, a riot will most likely begin.
Another cause can be when one of your Armed Guard shoots in front of multiple prisoners. Gangs will often provoke a riot if one of their members are being attacked by another prisoner or guard. Gang members will immediately initiate a riot if their leader is killed.
How It Works
The temperature of your prison must be at least 33% before a riot can start. 10 Prisoners must be rioting before a prison-wide riot status can occur.
Once a riot begins, the overlay for the emergency will appear as seen in the image above. The only information given to you is the number of prisoners that are rioting.
During a riot, prisoners involved will begin destroying any Objects that are near them, or they will attack any Staff member they see. Additionally, rioting prisoner will rarely attack each other unless they have violent traits. It is assumed that prisoners want to escape during a riot, and attacking each other will make that difficult to accomplish. Prisoners will also ignore all rules of the prison and will begin stealing Contraband from Rooms, and using them against Staff members.
As a riot gets progressively stronger, guards, dog handlers, armed guards and Snipers will start losing control, and sectors of your prison will start getting claimed by your prisoners. When a sector is overtaken by prisoners, it will highlight red in color, and your guards, dog handlers and Snipers will refuse to enter it in fear for their lives while armed guards will still enter. If Fog of War is enabled, you will eventually lose visibility of that sector.
The only effective way to fight a riot after it has gone out of control is to call in Riot Police. These units carry riot shields which protect against most attacks, making them tougher and more confident than guards, dog handlers and armed guards. Riot police are controllable units that need to be told where to go, and will start fighting prisoners who are misconducting. Once all but maximum 3 rioting prisoners have been knocked unconscious, surrendered or killed, the riot will end.
A good strategy to use before a riot begins is to use the Bangup Emergency Action. Doing this will tell all of your willing prisoners to go back to their cells, which will lessen the potential power of any riot. It will also help your guards, dog handlers and armed guards because they won't have to deal with every prisoner at once, they can just handle them one by one in their cells.
Soldiers aka National Guards arriving to prison to take control over it