Justin Bieber fever hit the UK last night as fans caused chaos outside the X Factor studios.
A huge crowd gathered outside in the freezing cold to try at catch a glimpse of Bieber who was singing on the show.
After several hours fans were rewarded with a glimpse of the star, when he popped his head out of the door and came and shook hands with fans.
Bieber performed a mash-up of his top songs Baby and Somebody to Love and said a special hello to judge Cheryl.
He also gave the contestants some advice to help them cope with being famous.
Crowds of Bieber fans outside the X Factor studios.
He said singers should always remember who they are.
After he finished singing the Canadian pop star said thanks to all of his fans for all of their support.
Justin's on a short visit to the UK. But don't worry, he'll be back in the UK next year as part of world tour.