What you need:
1. Innocent looking person (possibly an old man)
2. One of them scary looking gangbangers
3. A car and a public area.
Have a person drive their car up to a crowded area. Have the thug walk in front of the car as if he were crossing the street (have him walking extra slow like he don't give a f***.) Have the person honk at the thug to make him pissed off. Then have the thug walk over to the driver's seat, open the door, punch the dude in the face, throw him out of the car, get in the car and take off. Everyone in sight will then proceed to freak the f**k out.
(We definitely did this with my friends and filmed it, however won't submit it because of the terms, but it got a lot of attention and was awesome.)