  Context. The CoRoT mission, a pioneer in exoplanet searches from space, has
completed its first 150 days of continuous observations of ~12000 stars in the
galactic plane. An analysis of the raw data identifies the most promising
candidates and triggers the ground-based follow-up. Aims. We report on the
discovery of the transiting planet CoRoT-Exo-2b, with a period of 1.743 days,
and characterize its main parameters. Methods. We filter the CoRoT raw light
curve of cosmic impacts, orbital residuals, and low frequency signals from the
star. The folded light curve of 78 transits is fitted to a model to obtain the
main parameters. Radial velocity data obtained with the SOPHIE, CORALIE and
HARPS spectro-graphs are combined to characterize the system. The 2.5 min
binned phase-folded light curve is affected by the effect of sucessive
occultations of stellar active regions by the planet, and the dispersion in the
out of transit part reaches a level of 1.09x10-4 in flux units. Results. We
derive a radius for the planet of 1.465+-0.029 R_Jup and a mass of 3.31+-0.16
M_Jup, corresponding to a density of 1.31+-0.04 g/cm^3. The large radius of
CoRoT-Exo-2b cannot be explained by current models of evolution of irradiated
planets.
