Monte Carlo simulation techniques have been used to model the sequence of events that are generated by the interaction of ionising radiations with pure liquid water. Those events include (i) the energy depositions that occur through the ionisation and the excitation of water molecules, and (ii) the relaxation pathways and the ultrafast reactions of the subexcitation electrons, of the transient water anions and cations, and of the excited water molecules. The subsequent homogeneous and nonhomogeneous chemical reactions that take place along the radiation tracks when the radiolytic species diffuse into the medium are treated in the following paper (Frongillo et al., 1998a). The principles and the physical laws on which our simulation codes are based are explained in some detail to identify their strengths and weaknesses and to point out the main approximations that are made. The incident particles that are considered are electrons with energies up to 150 keV and protons of less than 300 MeV. The specificity of the liquid phase for the interaction of those particles with water is accounted for in several ways, through appropriate choices of cross sections as well as ionisation and excitation potentials. Our simulation codes, called TRACPRO and TRACELE, are used to provide information on intermediate steps of the action of the radiation which are not readily observable. For example, the yields of the initial radiolytic species on the femtosecond time scale are determined and the relative contributions of their various formation channels are given. The energy distribution of the secondary electrons is also shown.