Low plasma density discharges at high longitudinal electric fields (E0⩾1 V m-1) in the ISTTOK tokamak revealed characteristic features for the runaway (RA) generation process. The increase in soft x-ray emission was detected at the onset of the RA regime. The relaxations of the plasma parameters, a typical satellite effect of the instability driven by the RA electrons, were observed in experiments. In some discharges instability caused minor disruptions. Simultaneous determination of the RA electron characteristics and reconstruction of the electron temperature temporal evolution has been performed using self-consistent 0-D calculations of the plasma transport on the basis of the measured plasma parameters with the inclusion of the RA generation process. The kinetic energy as well as the confinement time of RA electrons was evaluated. The modelling procedure developed was applied in the investigation of a possible RA generation process at the thermal quench in large-scale tokamak disruption.