Underlying event studies in inelastic pp collision events with the ATLAS detector

The first measurement of the charged particle flow in inelastic pp collision events with the ATLAS detector is described. The analysis is based on minimum-bias events collected at centre of mass energies of $900,mathrm{GeV}$ and $7,mathrm{TeV}$. The density of charged particles and their transverse momentum sum is measured in different regions of azimuthal angle defined with respect to the leading charged particle in the event. The data show a higher underlying event activity than predicted by different Monte Carlo models and tunes.