The NFL free agent -- who was most recently playing offensive tackle for the Seattle Seahawks -- was busted Saturday at LAX around 2 PM, where law enforcement sources tell us ... he allegedly tried going through TSA with a gun in his luggage.

We're told this happened at Terminal 6, and that during the standard screening ... one of Brown's bags was found to have a firearm inside, which our sources say was unloaded.

Of course, the cops were called immediately ... and our sources say Brown was taken into custody. He was actually recorded being escorted to a police vehicle by a photographer on the scene, and sure enough ... there's DB in handcuffs as he's led along by a few cops.

The charge we're told he's being booked on ... possession of a concealed firearm. Brown's on his way to a nearby LAPD jail for processing.