A security tag assembly and security system associated therewith prevent the unauthorized removal of merchandise from a given area, such as a retail establishment. The security tag assembly comprises a base and at least one attachment member removably connected in an operative position on the merchandise being protected. A locking assembly including a locking member is structured to restrict detachment of the attachment member and is forcibly disposed out of movement restricting relation to said connector member when the base and the attachment member are separated. A detachment assembly and a data registering processor may include a two-way communication link which facilitates processing of the merchandise data disposed on the security tag assembly, such that the purchasing of the merchandise and the removal of the security tag assembly can be effectively accomplished without jeopardizing the security of the merchandise being sold.