We demonstrate the channel assignment in a metropolitan wireless multi-radio mesh network with directional antennas. A channel assignment approach is presented using suitable algorithms with key components the interference model, the link ordering and the channel selection metric. We have implemented and evaluated the proposed channel assignment in an actual metropolitan mesh network which covers an area of 60km2 in the city of Heraklion and consists of 14 nodes, among which seven are core nodes with up to four 802.11a wireless interfaces each, and one for management and monitoring. Finally, we developed a Java application to demonstrate the channel assignment process in a metropolitan mesh network in a graphical environment.