The Monday night robbery at Zaventem airport just before 8pm was "one of the biggest" ever, said a spokeswoman for the Antwerp World Diamond Centre (AWDC), the global dealers' syndicate.
A gang of eight hooded thieves drove onto the runway in two black vehicles with blue police-like markings, Brussels prosecutors' spokeswoman Anja Bijnens told a press conference.
They forced their way through security barriers and sped towards the Helvetic Airways passenger aircraft about to take off, forcing open the cargo hold to reach gems — rough and cut — that had already been loaded, she said.
Bijnens said the thieves were wearing police uniforms and carrying machine guns, adding: "They wanted to pass themselves off as cops."
They seized at least 120 packages, which was only part of the shipment, she said.
The pilot, co-pilot and staff from a Brink's armoured car that transported the gems were held up but "no shots were fired and no-one was injured," Bijnens said of a robbery that was over "within minutes."
She said the thieves made off at high speed through the same gap in the security cordon they had opened in front of unsuspecting ground staff and travellers, adding that the passengers on board the plane "saw nothing" and that the aircraft, bound for Zurich, did not leave Brussels.
The Helvetic Airways plane was operated by Swiss International Air Lines.
The Helvetic flight, scheduled to leave Brussels for Zurich at 8.05 pm, was cancelled, Swiss spokesman Mehdi Guenin told ATS, the Swiss news agency.
The airline is conducting an investigation into what happened.
Helvetic, which leases three of its seven planes to Swiss, is owned by Patinex AG, an investment company owned in turn by Swiss financier Martin Ebner and his wife.
Meanwhile, Belgian police said they found one of the getaway cars, abandoned and burned, west of Brussels.
A specialist Belgian prosecutors unit dealing with organized crime is "pursuing all lines of enquiry," Bijnens said, and is collaborating also with Swiss authorities.
"This was not a random robbery," she stressed. "It was well-prepared — these were professionals."
According to the AWDC, the global diamond business is worth more than $60 billion each year.
Some $200 million worth of stones move in and out of Antwerp every day, the spokeswoman added.
The diamond community was "shocked by the brutal heist," said Caroline De Wolf of the AWDC in a subsequent statement.
She said traders want "additional security measures" implemented at the Brussels airport.