Investigations of the Raman scattering spectra of PbZr_0.99Ti_0.01O₃ single crystals were performed over a large temperature range from 10 to 650 K and compared with those of pure PbZrO₃. The main modifications of the Raman spectra have been found to appear near the transitions between the antiferroelectric-intermediate and the intermediate-paraelectric phases. In particular, a polarisation effect of the light scattering in the para-electric phase has been detected and attributed to the special role of Ti ions introduced in the host lattice of lead zirconate. These ions can be regarded as defects breaking the local symmetry and leading to the creation of polar clusters in the para-electric phase. Below a defined temperature in this phase, a long-range interaction appears and causes cluster ordering. The change in the Raman spectra and in the light scattering intensity and polarisation is analysed and connected with the dielectric and pyroelectric properties investigated on the same sample.