Phasor Measurement Units (PMU) are widely used in the real-time monitoring of electrical power systems. This work presents the design and construction of a low-cost prototype of a PMU for three-phase distribution Power Systems according to the IEEE standard C37.118.1. This system allows to estimate phasors, frequency, rate of change of frequency and symmetric components with the time label corresponding to each value estimated in a three-phase network of a low power system. All these parameters are synchronized using the signals from a Global Positioning System-GPS as a time reference. The system uses as base a development platform controlled by a Digital Signal Processor (DSP) and the estimation of the synchrophasors is done using the Discrete Fourier Transform (DFT). Firstly, a phasor estimation method is presented, after the implemented hardware and software is described. Finally, the results obtained from the measurements are presented in order to verify the correct operation of the equipment. Additionally, operation and communication tests are performed to validate the prototype according to the IEEE C37.118.1 standard.