The fire broke out early Tuesday morning in the so-called Oxy building on Place De Brouckère.
Several construction workers were trapped in an elevator when the fire spread through the elevator shaft to several floors of the building.
Six bodies have been found, a spokesman for the city's mayor told the AFP news agency.
Two people with serious burns were taken to hospital, while one firefighter received treatment at the scene.
The cause of the fire is unclear. The Oxy building is under renovation and will in the future house apartments, restaurants and a rooftop bar. Over 200 people are working on the construction.
Inspectors must investigate whether all relevant rules have been followed.
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