  A measurement of the underlying activity in scattering processes with
transverse momentum scale in the GeV region is performed in proton-proton
collisions at sqrt(s) = 0.9 TeV, using data collected by the CMS experiment at
the LHC. Charged hadron production is studied with reference to the direction
of a leading object, either a charged particle or a set of charged particles
forming a jet. Predictions of several QCD-inspired models as implemented in
PYTHIA are compared, after full detector simulation, to the data. The models
generally predict too little production of charged hadrons with pseudorapidity
eta < 2, p_T > 0.5 GeV/c, and azimuthal direction transverse to that of the
leading object.
