Abstract: A portable mechanical burglar alarm for monitoring doors and windows includes a base plate having a back surface provided with a plurality of suction cups for releasable securement adjacent a door or window. A pair of bells are secured in spaced relation on a front surface of the base plate and a pivotal oscillating clapper arm is disposed therebetween. A spring powered bell ringing mechanism is enclosed by a housing on a front surface of the base plate. A trigger mechanism within the housing is operative to actuate the bell ringing mechanism upon disengagement of an adjustable abutment stop member from contact with a door or window frame upon opening of the door or window. A retaining mechanism within the housing prevents unauthorized resetting of the trigger mechanism subsequent to activation, to prevent the ringing bells from being silenced.