Whether it be a police protest, political protest, or some or type of large gathering, as an officer, you may be detailed to attend within the vicinity to maintain a police presence.
As a cop, your role will be to standby and not let things get too heated and out of hand.
There may be small squabbles within the group, traffic problems that cause disputes among citizens, and problems between the crowds and nearby businesses.
Just do your best to become a mediator when these small disputes take place.
But, there will be times when these small disputes snowball into a much larger problem. At that point, you could be looking at a much more serous problem.
Generally speaking, most people are respectful of one another. However, sometimes you will see a change in behavior that takes place when individuals are gathered in large crowds.
It's a mob mentality that takes over, and for some reason, ordinary people who would normally be law abiding citizens become engulfed in the chaos, and act, well, stupid.
They'll do dumb things that they may not normally do. Light cars on fire, loot shops, smash windows etc etc. Basically it becomes a riot.
Sometimes this can happen because of the nature of the protest. Perhaps it's a police protest and the crowds are charged up because of some beef they have with the Police.
Maybe it's Political and it had more to do with a head of state and the way that things are being run which causes violent outburst ( look at what's been going on in Greece).
Or, it may not even be a protest. It could just be a large gathering that disintegrates and becomes a riot. A good example of this is the Stanley Cup riot which took place in Vancouver.
From watching the television footage, you can see that there were massive crowds of people in the downtown area of Vancouver, BC, watching game 7 of the Stanley Cup finals in 2011.
When Vancouver lost the game, several people in the crowd began trashing portions of the city. Below is a video from someone who was in the street when the Vancouver riot took place.
Individuals would not normally behave like this in the middle of a downtown core in front of millions, but hidden within a large number, people gain a sort of "bravery" and for whatever reason some people basically act like asses.
As a cop, you hope that gatherings / protests don't degrade into this sort of chaos, but unfortunately, riots are a reality and do take place.
Just realize that crowd control can be part of your job description and could arise at any time.