The effect of preexisting discontinuities on hydraulic fracture propagation in low permeability sandstone blocks was studied by means of Acoustic Emission (AE) and Ultrasonic Transmission (UT) techniques. The blocks were loaded in a polyaxial test frame to representative effective in-situ stress conditions. We used rock samples with saw cut discontinuities oriented orthogonally to the expected direction of fracture propagation. Hydraulic fracturing was initiated by injection of silicon oil or glycerin into a borehole drilled offset from the center of the block. Detailed analysis of combined AE localizations and UT results allowed identification of various stages of hydraulic fracturing, including initiation, interaction with the preexisting interface, propagation away from the interface, and closure of the fracture. It has been found that the onset of borehole pressure breakdown closely correlates to the time when the hydraulic fracture crosses interface, and not with the fracture initiation. Injection of high-viscosity fluid results in wider fractures and higher breakdown pressures, while injection of low-viscosity fluid results in narrower width of fractures developed at lower injection pressure. These results also give a reference for understanding and interpretation of microseismic monitoring data recorded in the field.