London: Teen sensation Justin Beiber had an embarrassing wardrobe malfunction in front of a house-full crowd at his Liverpool concert after he stepped on stage without zipping up his pants.

The zip on the teenagers trousers gaped open as he greeted the screaming 11,000-strong fans that had been waiting for their idol to arrive, a website reported.

But the 17-year-old remained unaware of the error as he sang and danced his way through the two-hour-performance.

Prior to his entry at the concert, Beiber was trapped in his hotel, which was surrounded by thousands of his fans.    

The fans, most of them teenage girls, crowded all roads to the hotel and the Canadian star was warned by police he could be arrested if he attempted to venture outside  and would be charged with "inciting a riot".