BRUSSELS—Three months after masked robbers stole more than $50 million in diamonds from the belly of a plane about to leave Brussels Airport, police in France, Switzerland and Belgium detained more than two dozen people and confiscated a "significant" stash of diamonds and cash.

The sweep began Tuesday when one person was arrested in France and eight others were brought in for questioning in Switzerland, according to Swiss and Belgian authorities.

Then, in early-morning raids Wednesday, some 250 Belgian police officers arrested 24 people in the Brussels area, said Anja Bijnens at the state prosecutor's office in Brussels.

The person arrested in France is believed to have been one of eight heavily armed robbers, disguised as police, who sped onto the tarmac of the Brussels Airport on Feb. 18 in two vehicles equipped with blue flashing lights.

Without firing a shot, they stole 120 packages of diamonds that had just been loaded into the hold of a Fokker-100 regional jet bound for Zurich, then escaped back through a hole cut in the perimeter fence.