As computing and communications increasingly pervade our lives, security and protection of sensitive data and systems are emerging as extremely important issues. Networks-on-Chip (NoCs) have appeared as design strategy to cope with the rapid increase in complexity of Multiprocessor Systems-on-Chip (MPSoCs), but only recently research community have addressed security on NoC-based architectures.In this paper, we present a monitoring system for NoC based architectures, whose goal is to help detect security violations carried out against the system. Information collected are sent to a central unit for efficiently counteracting actions performed by attackers. We detail the design of the basic blocks and analyse overhead associated with the ASIC implementation of the monitoring system, discussing type of security threats that it can help detect and counteract.