BRUSSELS — Robbers dressed as police and armed with machine guns have stolen 120 parcels of diamonds worth an estimated $50 million from the runway of Brussels Airport in one of the biggest heists the industry has seen.

Two vehicles carrying eight men drove up to a van belonging to security firm Brinks, which had just finished loading the diamonds into a Swiss passenger plane, on Monday evening, officials said. Police were still investigating and searching for the haul on Wednesday.

The men, who were masked but also wore police uniforms, did not fire a single shot and the entire heist took less than five minutes, said Ine Van Wymersch, spokeswoman for the Brussels prosecutor's office.

They escaped and are now being hunted by police.

"They were well prepared," Van Wymersch said. "There were passengers on the plane but they saw nothing of what was going on.

Prosecutors said they could not yet comment on how much the diamonds taken were worth or to whom they belonged.

Belgian state broadcaster VRT reported the total value was 350 million euros ($467 million). A spokeswoman for the Antwerp Diamond Center said the value may be closer to $50 million.

"In any case it's one of the biggest robberies we've seen," she said.

Antwerp has been a leading diamond center for centuries, with some eight in every 10 rough diamonds, and five in every 10 polished diamonds, passing through it.

Both vehicles sped off after the robbery and one was later found burnt just outside of Brussels.

The robbers managed to get onto the tarmac after breaking through the fence that surrounds the airport, an airport spokesman said.