Government agencies and military are seeing a rise in drones used for terrorism, destruction and espionage. As recently as a decade ago, UAV technology has been used only by US military. Today, dozens of countries manufacture and operate military-grade drones. Drone technology has been made available around the world. Anyone can purchase a drone from an online retailer. Western military operations as well as critical infrastructure protection agencies experienced multiple drone incidents in the last years, ranging from the use of weaponized drones by ISIS to drones flying over airports or drones breaching airspace of other critical infrastructure. The emergence of threats caused by unfriendly or hostile drones requires a proactive drone detection in order to decide on appropriate defence actions. In this contribution, an open architecture of a UAV detection system including decision support for counter-action is presented. The system is composed of multiple deployable sensor stations, an operation center comprising the operational picture display and the decision support component, and a communication bus consisting of a message-oriented middleware connecting the sub-systems and components. The architectural design specifies the sub-systems, their constitutive components, the information and control flow between the components, the protocols used for data and information exchange, the functionality and responsibility of each component, and the functional parameters. The designed architecture provides a blueprint for a UAV detection and defence actions decision-making system which will allow public protection agencies and military to react timely against threats caused by hostile drones.