A tamper indicating closure cap or overseal comprises a top wall, and an annular top wall skirt attached to a periphery of the top wall. A locking member is attached to the top wall skirt so as to be movable between:i) a first position projecting inwardly with respect to the top wall for application of the closure cap or overseal to a container opening and/or to a container closure plug andii) a second position projecting inwardly and upwardly towards the top wall, thereby to lock the closure cap or overseal to the container closure and/or closure plug.