Justin Bieber's free concert draws 15,000 fans

Justin Bieber staged a half-hour mini-concert in Oslo Wednesday that attracted 15,000 fans, who mobbed the Opera House roof to watch the "secret" outdoor show.

After Norwegian Broadcasting reported Bieber would hold the free concert at 9:20 p.m. local time, thousands of fans converged hours beforehand, gridlocking traffic and sparking a heavy police presence, said the English-language Norwegian site Views and News from Norway,

Fans first descended on the main airport and then at the city's Grand Hotel on Tuesday, the site said. As crowds swelled Wednesday, Bieber tweeted, "Please listen to the police. I don't want anyone getting hurt."

He said the six-song show would be taped for an NBC TV special, "Justin Bieber: All Around the World," to air June 21.

The concert marked Bieber's first public appearance since an altercation with a photographer in California on Sunday afternoon.