  We present a multi-wavelength study of GRB 081008, at redshift 1.967, by
Swift, ROTSE-III and GROND. Compared to other Swift GRBs, GRB 081008 has a
typical gamma-ray isotropic equivalent energy output (10^53 erg) during the
prompt phase, and displayed two temporally separated clusters of pulses. The
early X-ray emission seen by the Swift/XRT was dominated by the softening tail
of the prompt emission, producing multiple flares during and after the
Swift/BAT detections. Optical observations that started shortly after the first
active phase of gamma-ray emission showed two consecutive peaks. We interpret
the first optical peak as the onset of the afterglow associated with the early
burst activities. A second optical peak, coincident with the later gamma-ray
pulses, imposes a small modification to the otherwise smooth lightcurve and
thus suggests a minimal contribution from a probable internal component. We
suggest the early optical variability may be from continuous energy injection
into the forward shock front by later shells producing the second epoch of
burst activities. These early observations thus provide a potential probe for
the transition from prompt to the afterglow phase. The later lightcurve of GRB
081008 displays a smooth steepening in all optical bands and X-ray. The
temporal break is consistent with being achromatic at the observed wavelengths.
Our broad energy coverage shortly after the break constrains a spectral break
within optical. However, the evolution of the break frequency is not observed.
We discuss the plausible interpretations of this behavior.
