The law enforcement and emergency services would give you information once something begins, but these protest groups use text and social media to plan, recruit and coordinate. I am not sure HAM or other radio transmissions will give you advance warning.
Exactly. I think people underestimate the power and reach of activist and protest groups on social media. These groups are very active and send out event notifications, weeks and days in advance.
Just today, I received a notification for an oil line construction protest. Which although is not my interest, is an in area where if I were to be on the day of the event, I would avoid completely due to being held in a very busy and high traffic area. This means the various LEO units are not going to stand around and let the traffic be disrupted for long. And so I would imagine, things will get somewhat tense and ugly for awhile - but not to the point of riots and properties being damaged or burned down.