Prince Jackson -- Face Off with Cops ... GUNS OUT

Prince Jackson was fully armed yesterday during a brief standoff with the LAPD ... wearing a revealing bright green Euro-trash muscle tee that completely exposed his boyish pythons.

The photog on scene says 16-year-old Prince -- who's only been driving a few months -- was given a warning for illegal strobe lights on the grill of his new truck. Prince removed them and went on his way.

It's a far cry from the boy Michael Jackson shielded for years.