Experimental results on minor and major disruption events in discharges with runaway electrons in the small tokamak ISTTOK and corresponding numerical analysis are presented.Using the experimental data from the soft X-ray diagnostic the image of the plasma column in the presence of runaway electron instability and disruptions was reconstructed. Analysis of the emission produced during disruptions enabled to locate the most probable zones of the interaction of energetic electrons with plasma surrounding surfaces. Numerical analysis of the runaway instability allowed determining the runaway electrons characteristics, such as energy and density. On the basis of this analysis the level of electrostatic turbulence due to the instability was estimated.