A mains-wired monitoring and security system comprises an electric supply mains network (10) having connected thereto a control unit (Figure 1), a plurality of detecting units (Figure 2), and a plurality of actuating units (Figure 3). Each unit includes associated counter means (5, 15,29) controlled in synchronism with each other. Each detecting unit transmits a coded signal (f2) along the mains network when an associated sensor (14) detects an alarm condition and its associated counter means (15) simultaneously counts a specific number N. At least one actuating unit activates an associated actuator (37) in response to said coded signal if its associated counter simultaneously counts the number N. The central unot visually displays said number N, which is common to at least one detecting unit and at least one actuating unit, in response to said coded signal.