The barrier (200) consists of a prefabricated concrete block (300) fitted with a panel (400), e.g. of mesh, cables or bars, that is held in an upper surface groove (330) by wedges (430). The block has lower surface recesses (312) to facilitate handling by a forklift truck, and adjoining blocks are connected to one another by a cable (500) passing through rings (550) on their inner surfaces to prevent the fence being breached by moving one of the blocks.