In Frostpunk 2, there is a clear distinction between rallies and riots. Rallies are usually organized group events that may be for the purpose of communicating or expressing demands. Riots are chaotic and out of control, often accompanied by violence and vandalism. Assemblies may proceed in an orderly manner, but riots may seriously affect order and development.
Rallies and riots are a direct result of your relationships with factions and their interrelationships.
Rallying is caused by increased enthusiasm and good relations between you and a faction.
Riots are caused by increased enthusiasm and negative relationships (with you or other factions).
In the main story, only two factions are able to hold rallies and initiate riots. These two factions sit in the middle of your relationship bar and are the only ones whose passions can be viewed. For example, if you choose the Faith route, the two factions would be the Faith Keepers and the Evolvers; similarly, the Order route would include the Stalwarts and the Pilgrims.