In this paper we present the results of a detailed numerical investigation of plasma formed at the preionization stage of extreme ultraviolet (EUV) lasers based on nanosecond capillary discharges. Despite the general consensus that preliminary ionization is one of the features that have originally allowed creating stable and efficient lasers operated in argon-filled capillaries, little attention has been paid to the observed sensitivity of their performance to the properties of the preionizing current pulse. The goal of present studies was to obtain basic description of preliminary plasma state that could be used for interpretation of available experimental data on the subject. The numerical model was based on the hydrodynamic 'fluid' approach coupled with the heat transfer, the continuity and the Navier-Stokes equations. Preliminary discharge dynamics for conditions typical of an argon EUV laser is illustrated in detail, starting from the initial breakdown, taking the form of a fast ionization wave, to formation of a self-sustaining nonequilibrium plasma column. It is shown that a few microseconds after application of the prepulse a concave gas density profile is formed that can potentially be a factor influencing plasma compression and emission during the main stage of a capillary discharge.