I just say forcefully "Excuse me, but I was here hirst." If they insist on name calling (You white bitch), I repeat "I was here first." If they still don't take the hint, I start a running dialogue "I was here first. These people think they can just be impolite and run the world. Who are they, in charge of the world. Totally rude." Of course, by that time they are screaming at me to shut up, but I just keep going on.
From my experience, gangs of young African-American females are one of the scariest things in the world. I don't know why.