An insider says Chris knocked Frank unconscious with a series of blows to the head, and Frank's friends had to carry him inside where they called the cops.

A spokesperson for the L.A. Sheriff's Department says, "Chris Brown and the victim were involved in an altercation over a parking space. The altercation allegedly led to Chris Brown punching the victim." The spokesperson added that Frank has indicated that he wants to press charges against Chris. This could put Chris in some SERIOUS hot water, because he's still on probation for beating up Rihanna.

Yesterday, Frank Tweeted, "Got jumped by Chris and a couple guys. LOL . . . Cut my finger now I can't play with two hands at the Grammys."

While Chris Brown Instagram'd a painting of Jesus being CRUCIFIED with the caption, "Painting the way I feel today. Focus on what matters!"