  We calculate the Raman response contribution due to collective modes, finding
a strong dependence on the photon polarizations and on the characteristic
wavevectors of the modes. We compare our results with recent Raman spectroscopy
experiments in underdoped cuprates, $La_{2-x}Sr_xCuO_4$ and
$(Y_{1.97}Ca_{0.3})Ba_2CuO_{6.05}$, where anomalous low-energy peaks are
observed, which soften upon lowering the temperature. We show that the specific
dependence on doping and on photon polarizations of these peaks is only
compatible with charge collective excitations at finite wavelength.
