[en] Results are presented obtained in a kinematically-full experiment carried out at the JINR phasotron with a three-arm spark spectrometer for studying mechanisms for the (p, 3p) reaction of knock-on of proton pairs from the carbon nucleus by 660-MeV protons. They were used to find differential cross sections for reaction, energy spectra of secondary protons, and momentum spectra of the residual nuclei for three experimental geometries. The cross section for the fourth experimental geometry with initial proton energy of 350 MeV was estimated. The experimental distributions are compared with the relevant calculations for different reaction mechanisms. 8 refs., 7 figs