Source, in full
What are you going to do with your gun? Because if you try to shoot at me, or you try to take this gun, I'm going to shoot back at you, swine ... You come from Georgia somewhere, you're downtown making $800 a month, and you come down here murdering and brutalizing black people in the black communities. They gave you sergeant stripes and I say you're nothing but a low-life, scurvy swine. A sharecropper from racist Georgia in the South somewhere. So if you draw that gun, I'll shoot back at you and I'll blow your brains out!
Notes from Wikiquote
Police officer: You ... you ... you ... you're just turning the Constitution around! | Newton: I'm not turning anything around. And I got my gun. What are you going to do with yours? | Confrontation related by Bobby Seale, in Seize the Time; The Story of the Black Panther Party and Huey P. Newton (1970)