I think its AGGRO minis that do a football riot style game, which may work if you're looking for a modern setting.
However I've neither played that or Gangs of Rome, so I can't speak for what would be the most suitable. A few companies do modern rioters though - which means there aught to be some system out there for them to be used in.
The location and type of riot would likely influence which rules to use. I.e. particularly if either side are using firearms, or if its expected to get bloody - rather than people just being knocked out or arrested. Or if you want to go off the deep end a bit, whether to include rules for Police snipers, or vehicles being used as weapons for instance.
Skirmish scale systems could work. Not every encounter would be a mass of protestors vs a wall of police. Crowds could break away into their own fights, or say the police may pursue a smaller group into an area outside the main conflict.
You may also want to look into things like temporary barricades. Say just piles of junk which can hold back individuals for a time, but are no use against vehicles. I'm also picturing things like crowd control barriers used by police - placed in a mesh pattern so they're far more stable.
Fire may play a big part in any game. Maybe scoring points for destroying property, or strictly used for creating temporary barriers. Not necessarily just used by the rioters (one iconic image of a police officer comes to mind...).
Rules for Journalists could be interesting too. Giving the rioters some sort of morale bonus, and taking points off of the police if they're attacked by them. The police of course may choose to attack them anyway...
Vehicles may not be necessary. Maybe just using them as terrain, barriers, spawn points for police, or as static emplacements - armed with water cannons, sound projectors, etc.
Besides police there could be civilians protesting against the rioters, or pacifist groups (who again sides would lose points for engaging, and otherwise would act as a barrier).
Just a stream of thoughts.