The Canadian teen heart-throb was jeered just before he took to the stage at Bangor, Wales.
Justin performed three songs on the main stage on Saturday and the boos were soon drowned out by the screams of his loyal fans.
30 Seconds to Mars' frontman Jared Leto defended the singer saying: "He's a young kid. He'll be able to charm the crowd."
Alicia Keys, Cheryl Cole, Florence and the Machine, MGMT, Faithless, and Dizzee Rascal also performed at the festival.
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