The incident happened around 9PM as the French singer Jain was about to go on stage.
A security official stepped onto the stage and asked everybody to leave using the emergency exits. No reason for the evacuation was given. People filed out of the building calmly and resigned to the upset of their evening's plans.
Brussels police say that a couple overheard a conversation during which somebody was warned not to go inside because the whole place would blow up. The couple alerted local police.
People evacuated from the AB gathered outside the old stock exchange building and on the historic market square. All pubs in the immediate vicinity had been cleared too.
A suspect was detained, but soon released.