And the staff helps her out!

The Heroes hottie - who just turned 20, so she's STILL underage! - was out clubbing at Hollywood hot spot Madame Royale Wednesday night before leaving in a taxi with friends...

Of course, not before creating a fuss! First a big guard blocked her from view, and then a club staff member shoved his coat in front of photogs as Hayden slipped into the cab. A big argument followed after! Damn, that Hayden's trouble.

Watch all the craziness play out in our vid below: