LOS ANGELES: Justin Bieber topped Billboard's "21 Under 21" list of the world's top young musical acts for the second year in a row, fending off stiff competition from British boy band One Direction.

Bieber, 18, "remains more powerful in the pop universe than ever", Billboard said, having released two No. 1 albums in the last year - holiday set "Under the Mistletoe" and his latest pop album, "Believe".

"Bieber is the supernova of teen pop stars with charisma, popularity and swagger to spare ... it's Biebs' world, and we're lucky to live in it," the music publication said.