The thieves broke through a fence yesterday evening to steal a reported £43 million worth of diamonds as they were being loaded from a security van onto an aeroplane, before escaping back through the same hole.
Police later found a burned-out vehicle close to the airport and are reportedly looking for eight men.
Caroline De Wolf of the Antwerp World Diamond Centre said: “What we are talking about is obviously a gigantic sum.”
An unnamed spokeswoman from the same centre was quoted as calling the robbery “one of the biggest” ever.
Brussels prosecutor’s spokeswoman Anja Bijnens said the thieves were masked and well-armed, however, no shots were fired and no-one was hurt during the raid.
Jan Van Der Crujsse, an airport spokesman, could not explain the security breach, stating: “We (the airport) abide by the most stringent rules.”
Van Der Crujsse said the robbers made a hole in the perimeter fence and drove up next to the Swiss passenger plane that was preparing to leave.