It happened just hours before the sold-out concert. Crew members were setting up the stage at the Downsview Park venue when it suddenly caved in.
Security officials quickly cleared out the area. One person died at the scene, another suffered serious injuries and two others were treated for minor injuries.
"When we arrived, we still had reports of someone trapped under the structures. So crews went into the structure, assisted with extricating that victim. Once it was realized there were no more victims to be rescued, we realized with an unstable structure we needed to remove ourselves from the area," described Tony Bellavance with Toronto Fire Services.
Investigators are looking into what caused the overhang to fall.
The Associated Press contributed to this report.