There is provided a portable fiber optic cable based intrusion detection system in which support posts are driven into the earth to define a perimeter. A portably mountable fiber optic cable attenuator is installed to each support post. The fiber optic cable is wrapped around a first compressible member of the attenuator and then looped through a second compressible member. A retractable rod having a cross arm there through is elevated through the second compressible member to form an upper loop in fiber optic cable. The retractable rod is then retracted leaving the remaining upper loop. An external force on the fiber optic cable diminishes the upper loop about the cross arm thereby creating a microbend in the fiber optic cable. A light detector senses a reduced light intensity, caused by the microbend, at the end of the fiber optic cable which triggers an alarm.