These can be anyone you start a fight with. To start a fight, go up to someone at press to shove them. This will make them mad and go into the fight sequence. Men are generally the only ones that will fight back. Try to stay off the road, maybe in a hidden alley way so that the police don't see it and break it up.
A really easy place to do this is at the back of Derek's docks at the South End of the map. The dock workers respawn, and there are no police to break up the fights. You will get a wanted level though.