Armed robbers stole diamonds worth 50 million euros ($67 million) while the gems were being loaded onto a plane at Brussels Airport on Monday evening, Belgian state broadcaster VRT reported.
Initial reports said the number was as high as 350 million euros but later adjusted the figure downward.
Two vehicles carrying four armed men drove up to a security van near the Swiss passenger plane, officials said.
"The men were armed and masked. There were no shots fired and nobody was injured," a spokeswoman for the Brussels prosecutor said.
Both vehicles sped off after the robbery and one of them was later found, said officials, who would not comment on what was taken.
The robbers managed to get onto the tarmac after breaking through the fence that surrounds the airport, a spokesman for Brussels Airport said.