Music
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Leto performed with his band 30 Seconds to Mars on Saturday at the Bangor festival, according to Just Jared.
Bieber's name was announced over loudspeaker ahead of Leto's performance, prompting the audience to boo.
"He's a young kid. He'll be able to charm the crowd," Leto told the audience.
Cheryl Cole, Rihanna and Dizzee Rascal were among the performers at the event.
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