NABLUS — Shooting into the air, dozens of masked gunmen associated with the al-Aqsa Martyrs Brigade cut short a rock concert by a popular Palestinian singer in Nablus Tuesday night. Defying the burning tires and chants against him, the singer, Ammar Hassan, was whisked onto stage 45 minutes late. He opened his show with a song about "Holy Jerusalem" in an apparent effort to appease the gunmen. Less than an hour later, however, guards ushered him off stage. Hassan became the idol of many Palestinians last year when he won second place in a 13-week American Idol-type competition on the Lebanese TV channel 'al-Mustaqbal.' Time not right The concert was the opening of a festival meant to bring normalcy to the largest city in the West Bank, hard hit by more than 4 1/2 years of Palestinian-Israeli violence. "This is the not time to have parties like this in Nablus," said one of the masked gunmen, who would not give his name. "We lost a lot of martyrs and lost a lot of friends, and this is not appropriate for Nablus." The interruption of the concert was the latest sign of growing chaos in the Palestinian areas. The concert ended with a brawl between opponents and supporters of the show throwing chairs at each other. Then shouting erupted between armed university guards and policemen, but no shots were fired and no injuries were reported.