The 'Boyfriend' singer has kicked off his brand new world tour in North America after months of anticipation by his fans across the world.
Katy Perry And The Saturdays Watch Justin Bieber's Los Angeles 'Believe' Tour Show
The stars shared pictures of Justin with their fans as they admitted to having "Bieber fever".
Katy Perry and The Saturdays were among some of the stars who have tweeted their excitement at getting to watch Justin Bieber bring his 'Believe' world tour to Los Angeles last night.
The 'Boyfriend' singer performed his second consecutive night in the California city yesterday (3rd October), and both 'Teenage Dream' singer Katy and UK girl band stars Frankie Sandford and Mollie King shared pictures of their Bieber experience with fans on Twitter.
"WE ARE BELIEBERS!" Katy Perry tweeted alongside a picture of her and friends dressed up in Justin's trademark purple hoodie.
"Last night I saw the Black Keys, tonight I saw Bieber. I think my musical week is complete," Katy later added, after which Justin himself retweeted her post.
See Katy Perry getting into the spirit of Justin Bieber's 'Believe' world tour at his concert this week in Los Angeles below (Credit: Twitter):
The Saturdays have been out regularly in Los Angeles over the summer while preparing to launch their music career stateside, and band members Frankie and Mollie headed to the Staples Centre to watch the 'Never Say Never' star live.
"Is it a bird, is it a plane, no it's @JustinBieber!!!" Mollie tweeted, before adding: "Bieber Fever!! Oldest people here by a MILE! Hahaha! I feel ancient!"
Check out a picture of The Saturdays stars Mollie King and Frankie Sandford watching Justin Bieber perform live below (Credit: Twitter):
'As Long As You Love Me' singer Justin later tweeted fans to thank them for the positive reception in California this week, before he moves on to perform more shows this coming weekend.
He posted: "LOS ANGELES!! I LOVE YOU!! THANK YOU!! #BELIVETOUR …. next stop!"
Earlier this week it was announced that Justin will be one of three special guest performers at the Victoria's Secret Fashion Show in New York City on 4th December.
Justin Bieber is touring in support of his album 'Believe', which was released earlier this summer to massive sales worldwide.