  The first measurements from proton-proton collisions recorded with the ATLAS
detector at the LHC are presented. Data were collected in December 2009 using a
minimum-bias trigger during collisions at a centre-of-mass energy of 900 GeV.
The charged-particle multiplicity, its dependence on transverse momentum and
pseudorapidity, and the relationship between mean transverse momentum and
charged-particle multiplicity are measured for events with at least one charged
particle in the kinematic range |eta|<2.5 and pT>500 MeV. The measurements are
compared to Monte Carlo models of proton-proton collisions and to results from
other experiments at the same centre-of-mass energy. The charged-particle
multiplicity per event and unit of pseudorapidity at eta = 0 is measured to be
1.333 +/- 0.003 (stat.) +/- 0.040 (syst.), which is 5-15% higher than the Monte
Carlo models predict.
