At the beginning of mission 3 The Escape, you will encounter Zeke, where you plan to go across the bridge. You are given here, the option of how you plan to break through the guards standing in your way, either by making yourself known, or hiding behind the nearby protesters and attacking the guards that way.
For Good Riot you will want to make yourself separated from the other people by walking in front of the protesters and attacking the guards, giving you a good karma boost and this trophy.