The singer's security team reportedly laid down the strict instructions to people in the front row of his "FutureSex/LoveSounds" tour.

A source said: "They were quite specific about what was and what was not allowed. We've never known anything like it. It was hysterical."

Despite the stern warnings, Justin - who kicked off his UK and European tour at Belfast's Odyssey Arena on Tuesday (04.24.07) - was joined on stage by two men.

The source added: "Justin looked a bit perturbed. Far from concentrated on singing he was looking all around as the action unfolded."

The men had splashed out on exclusive tickets for the SexyBack Dance Club - a private area right in front of the stage.

It was reported they mocked Justin before the singer's burly bodyguards forced them off the stage.

Justin, who is touring with hip-hop mogul Timbaland, is also set to perform in Glasgow, Manchester, Paris, Munich, Milan, Berlin and Amsterdam.