Wide-Area monitoring systems are a requirement in the current power systems in order to provide an appropriate monitoring, supervision and protection from undesired events. Electromechanical oscillations can provoke critical situations and affect the power transmission capability. This paper applies the Multi-Prony Analysis (MPA) to estimate the critical modes by using the tie-lines power measurements from Phasor Measuerement Unit (PMU)-data. The modes observation including an online sliding window give the frequency and damping variation allowing to detect and classify the range of the oscillation.