A releasable security tag for attachment to an article of merchandise includes: a locking mechanism with a release for attaching the tag to the article of merchandise; an EAS resonant circuit or an RFID circuit for responding to a first RF signal at a predetermined frequency corresponding to the resonant circuit or to the RFID circuit; an ambient RF energy harvesting circuit; a release signal detection circuit coupled to and powered by the ambient RF energy harvesting circuit and an electro-mechanical actuator electrically coupled to the release signal detection circuit. The electro-mechanical actuator releases the locking mechanism whenever the release signal detection circuit receives a release signal.