The paper describes a phasor measurement unit (PMU) prototype based on a synchrophasor estimation technique specifically conceived for active distribution networks and the experimental results obtained by its application during intentional islanding and reconnection tests of a set of urban feeders. As proved by the laboratory experimental characterization of the developed PMUs, very low values of Total Vector Error (TVE) and both root mean square (RMS) and phase errors of the synchrophasors estimates are obtained, as required by their application in distribution networks. Moreover, a substantial independence of the PMU performances from the distortion level of the input signal has been obtained. As illustrated by the application during the islanding experimental tests, aimed at assessing the capability of the urban distribution network to operate autonomously fed by a local 80 MW combined cycle power plant and to reconnect to the external grid, the information provided by the PMUs appears to be useful for the development of improved control and management systems.