We have realized dark spatial photovoltaic solitons at wavelength of 632.8 nm in Fe and Cu- doped planar optical waveguides in lithium niobate. It has been shown that for lowest TE modes the photovoltaic nonlinearity only partly compensated the light diffraction effect at the experimental conditions used. The development time of corresponding light- induced photovoltaic lenses ranged from about 0.1 s to 30 s depending on the waveguide sample. For the highest TE modes we observed the exact compensation of diffraction effects in the time scale of some minutes followed in some cases by the termination of dark solitons formed because of the light overdefocusing.