The propagation of high-power femtosecond light pulses in lithium niobate crystals is investigated experimentally and theoretically in collinear pump-probe transmission experiments. It is found within a wide intensity range that a strong decrease of the pump transmission coefficient at wavelength fully complies with the model of two-photon absorption; the corresponding nonlinear absorption coefficient is . Furthermore, strong pump pulses induce a considerable absorption for the probe at . The dependence of the probe transmission coefficient on the time delay between probe and pump pulses is characterized by a narrow dip (at ) and a long (on the picosecond time scale) lasting plateau. The dip is due to direct two-photon transitions involving pump and probe photons; the corresponding nonlinear absorption coefficient is . The plateau absorption is caused by the presence of pump-excited charge carriers; the effective absorption cross section at is . The above nonlinear absorption parameters are not strongly polarization sensitive. No specific manifestations of the relaxation of hot carriers are found for a pulse duration of .