A method and a tag for opening a powerless electromechanical lock are provided. The tag comprises a power source, a near field communication transceiver, an antenna connected to the transceiver, an proximity switch and a controller. The switch is configured to wake up the controller from a low power mode upon a detection of a predetermined signal. The controller is configured after the wake up to activate the near field communication transceiver and control the transceiver to transmit via the antenna wirelessly first operating power to the lock for communication and authentication, perform authentication with the lock and, provided that the authentication is successful, control the transceiver to transmit wirelessly second operating power to the lock for the lock to be set into an openable state.