The stolen gems, a mix of rough and cut stones, had arrived at the airport by road from the Belgian port city of Antwerp, the world's biggest diamond trading center, and were to be flown on a scheduled passenger flight to Zurich, an important transit point in the global diamond business.
A spokeswoman for the Brussels prosecutors' office told a news conference at the airport, known as Zaventem, that eight "heavily armed and hooded" thieves had driven onto the tarmac in two black vehicles with flashing lights, local media reported.
"This was not a random robbery. It was well-prepared - these were professionals," said the spokeswoman, Anja Bijens, adding that the robbers had worn police uniforms and carried submachine guns. No shots were fired and no one was hurt.
The theft delivers a blow to Antwerp's role as a diamond center at a time when the city, a diamond trading and cutting hub for centuries, is struggling to fend off a challenge from low-wage diamond cutters in India and elsewhere.
"The fact that this happened is a big problem for us. We have our number one position to defend. Security is obviously very important," said Caroline De Wolf, a spokeswoman for the Antwerp World Diamond Centre, an industry body that promotes the diamond business in Belgium. "We are shocked by the fact this could ever happen. We are all wondering: How is this possible?"
Diamonds traded in Antwerp last year had a total value of $51.9 billion, accounting for 80 percent of the world's rough diamond trade and 50 percent of trade in polished gems. De Wolf said diamonds from Antwerp had been targeted by thieves before but Monday's robbery was the biggest she could recall.
Helvetic Airways, an independent Swiss airline that operated the plane targeted in the robbery, said security for valuable cargo is normally the responsibility of the airport and the security company hired to transport the shipment to the plane. An airline spokesman in Zurich declined to comment further.
Diamonds purchased in Antwerp for either cutting or sale abroad are usually taken to the Brussels airport under police escort in armored security vans and the thieves took advantage of a brief gap in this tightly guarded procedure during the loading of cargo. No arrests have been reported. Police said they had found the burned remains of a vehicle believed to have been used in the robbery near the airport Monday night.
A statement issued by the airport said the robbers had "entered the premises of the airport aboard two vehicles that had crossed the fence." The Zurich-bound flight, saidthe statement, had been canceled. It gave no details of how the vehicles had crossed the security fence and barriers.
The spokeswoman for the Antwerp diamond center said she believed the robbers had cut a hole in the fence and then raced to the Zurich-bound aircraft to grab gems from the cargo hold while passengers onboard waited for takeoff.
Bijens, the prosecutor's spokeswoman, said the thieves seized at least 120 packages of diamonds but added that not all the shipment had been stolen.