A doping official says bodybuilders just grabbed their gear and ran off when he came into the room.
"I have never seen anything like it and hope never to see anything like it again," doping official Hans Cooman said Monday.
Twenty bodybuilders were entered in the weekend competition.
Cooman said the sport has a history of doping "and this incident didn't do its reputation any good."
During testing of bodybuilding events last year, doping authorities of northern Belgium's Flanders region found that three-quarters of the competitors tested positive.