The former Spice Girl sat on top of her car and addressed the crowds with a megaphone.
"La Halliwell come over all Marilyn gives us all a master-class in how to make an entrance..." wrote the show's official Twitter feed.
Halliwell is a guest judge for the first round of filmed auditions, which are taking place in Liverpool.
The singer has been heavily tipped to be appointed as the permanent replacement for Kelly Rowland.
Comedian Peter Kay will reportedly warm up the crowd on the first day of filmed auditions after apparently making an approach to producers of the show for the slot.
Photo gallery - X Factor auditions in Liverpool: