In response to the security threats faced by multi-area interconnected power systems, particularly various potential attacks, a security protection scheme based on an extended state observer is proposed. The proposed approach extends the state vector by introducing new variables, allowing disturbances and attacks to be treated as additional state variables within the system. Through the utilization of the extended state observer, separate estimations of interference and external attacks can be obtained, thereby enhancing the power system resilience against potential threats. Based on this, a threshold-based attack detection algorithm is proposed. In simulation experiments, the model of the multi-area interconnected power system is studied under various attacks such as step attacks, ramp attacks, and periodic attacks using active disturbance rejection control (ADRC). Based on the performance comparison between traditional PI control and ADRC, it has been demonstrated that the ADRC method exhibits significant advantages in improving system response speed and enhancing control accuracy.