  Distributions sensitive to the underlying event in QCD jet events have been
measured with the ATLAS detector at the LHC, based on 37/pb of proton-proton
collision data collected at a centre-of-mass energy of 7 TeV. Charged-particle
mean $p_T$ and densities of all-particle $E_T$ and charged-particle
multiplicity and $p_T$ have been measured in regions azimuthally transverse to
the hardest jet in each event. These are presented both as one-dimensional
distributions and with their mean values as functions of the leading-jet
transverse momentum from 20 GeV to 800 GeV. The correlation of charged-particle
mean $p_T$ with charged-particle multiplicity is also studied, and the $E_T$
densities include the forward rapidity region; these features provide extra
data constraints for Monte Carlo modelling of colour reconnection and
beam-remnant effects respectively. For the first time, underlying event
observables have been computed separately for inclusive jet and exclusive dijet
event selections, allowing more detailed study of the interplay of multiple
partonic scattering and QCD radiation contributions to the underlying event.
Comparisons to the predictions of different Monte Carlo models show a need for
further model tuning, but the standard approach is found to generally reproduce
the features of the underlying event in both types of event selection.
