Another said: "Heard two bangs and loads of sirens in Brussels, anyone know what's happening?"
Emergency services are believed to have attempted to resuscitate the attacker but he died at the scene.
Just days ago jihadis urged attacks on a street just metres away from the scene of tonight's incident.
A chilling post on the Telegram app called for vehicles to mow down pedestrians on nearby Rue Neuve - where a man wearing a fake suicide belt also triggered a bomb alert last year.
That incident began after reports of a person acting suspiciously near the City 2 Shopping Centre.
A Brussels police spokesman said at the time: "The man was arrested, but it turned out that his suicide belt was fake. It contained salt and biscuits."