Eyewitnesses tell TMZ ... Brit and Sam Asghari showed up with a bodyguard at JOEY restaurant in Woodland Hills which was packed with patrons, who immediately recognized her.

From the start ... the dinner was a bust for Britney. Patrons pulled out their cell phones and began shooting footage of her. Britney got pissed and things went south quickly.

Eyewitnesses say Britney became "manic," yelling and talking gibberish -- not another language, just unrecognizable speech. We're told Sam got visibly upset, abruptly stood up and stormed out the door.

Check out the video, obtained by TMZ, showing Brit sitting alone at the table and speaking incoherently.

Two minutes later, witnesses say, Britney got up and left the restaurant with her bodyguard, who soon returned to pay the bill.