In this paper, we present the design and fabrication of a compact, modular optical high-voltage sensor (OHVS) based on fiber Bragg gratings (FBG) for a.c. distribution and transmission lines. The proposed OHVS is composed by a stack of piezoelectric transducers that transfer mechanical strain to a sensing FBG. A prototype was tested in the laboratory and showed a maximum linearity error of less than 3% of full-scale range (FSR) for input voltages up to ${14\,\,{\rm kV}_{\rm rms}}$14kVrms with a signal-to-noise ratio of 55 dB, allowing measurements with a resolution of less than 0.2% of FSR. Transient response of the developed OHVS was preliminarily investigated, and a response time of less than 1 ms was observed. The results obtained allow us to conclude that the developed OHVS may also be used for the detection of transient events.