Most demos are peaceful, and there is no trouble. In some cases it will be clear in advance that a protest or demonstration is going to be violent but some can turn in a flash. If that happens, be careful, go back to your car and put on safety equipment. Use your common sense as soon as things look like turning nasty.
Tell-tale signs of possible violence:
The mood of the crowd changes. Frowns replace smiles. Chatter ceases. The crowd starts shouting angry slogans.
A group or groups of (mostly) men gather, often on street corners.
The Leader or leaders of a demonstration start yelling angry provocative statements or slogans.
AT THIS STAGE YOU SHOULD GET OUT OF THE WAY. Only come back when you have put on safety clothing.