In this study, a nationwide real-time monitoring system has been developed to monitor all electrical quantities and power quality (PQ) parameters of the electricity transmission network including its interfaces with the generation and distribution systems. The implemented system is constituted of extended PQ analysers (PQ(+) analysers), a national monitoring center for power quality (NMCPQ), and the PQ retrieval and analysis software suite. The PQ(+) analyser is specifically designed for multipurpose usage such as event recording and raw data collection, in addition to the usual PQ analysis functions. The system has been designed to support up to 1000 PQ(+) analysers, to make the electricity transmission system and its interfaces completely observable in the future. The remote monitoring, analysis and reporting interface, the map-based interface, and the real-time monitoring interface, as well as the Web applications run on the NMCPQ servers, can be accessed by the utility, large industrial plants, distribution companies and researchers, to the extent of the authorisation given by the transmission system operator. By activating sufficient number of analogue and digital outputs of PQ+ analysers, and equipping the NMCPQ with necessary hardware, analysis and decision making software, the proposed system can serve as an advanced supervisory control and data acquisition system in the future. The proposed monitoring concept can be extended to serve the needs of the modern electricity markets, by making various regulations, codes and instructions implementable.