Sorry if these are obvious comments but it depends on what you are trying to accomplish. Most of the time if you are trying to show the crowd size you'll want to get the camera up above your head or be standing on a bench or something otherwise it'll be hard to realize that it is more than a few people standing in front of the lens.

Also, most of the riot shots I've seen are very powerful if you can show both sides of the battle. ie. the crowd and the security teams that are trying to control them. Even better if you can get the emotions of the people on both sides. Bored cops or incensed rioters, etc.