Thank you.

Just a few thoughts:
Riots don't usually start without some escalation first.
Emotional-upset people make bad decisions.
Large groups of people together aren't always good for your health.

If you're at an emotionally charged event, just keep your wits about you. Funny how almost all good plans involve or require keeping your wits. Pay attention to how your group is behaving as well as any counter-protesters, third-parties, etc.
If it starts escalating, move away from the most angry parts of the group and the more tightly-packed parts. A crowd doesn't have to be angry to be bad for you, a panicked trample will be just as harmful as an angry one. Get to the edge and prepare to just leave. You may not have carried your sign as long as you wanted, but it may be time to call it a day and leave.

Edit to add: If you are older or infirm or just don't get around that well, try to avoid getting into the more tightly-packed groups to begin with, at all times, not just when things look a little dicey.

If the police are there, co-operate with them and try not to impede their progress if they move to manage traffic or confront troublemakers.