Justin Bieber still No. 1 on Billboard's '21 Under 21' list

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".

One Direction, whose five members range in age from 18 to 20, burst onto the U.S. pop scene just six months ago after first soaring to success in their native Britain.