1. A surveillance system for monitoring a plurality of vehicle license plates comprising: a portable license plate reader including a camera for automatically imaging a license plate and extracting a character string from the image for each of a plurality of moving vehicles that pass through a field of view of the camera without detection by the moving vehicles; a mobile surveillance unit positioned in proximity to the license plate reader for receiving the extracted character string from the reader, comparing each extracted character string with a list of target plate numbers, and generating an audible alarm and a visual display when a match is found; an operations center for communicating with the mobile surveillance unit to receive each extracted character string from the mobile surveillance unit and to update the list of target plate numbers stored at the mobile surveillance unit; and an alert system for transmitting an alert to one or more external enforcement systems.