  We present an X-ray observation of the eclipsing binary Algol, obtained with
the BeppoSAX observatory. During the observation a huge flare was observed,
exceptional both in duration as well as in peak plasma temperature and total
energy release. The wide spectral response of the different BeppoSAX
instruments, together with the long decay time scale of the flare, allowed us
to perform a detailed time-resolved X-ray spectroscopic analysis of the flare.
We derive the physical parameters of the emitting region together with the
plasma density applying different methods to the observed flare decay. The
X-ray emission from the flare is totally eclipsed during the secondary optical
eclipse, so that the size of the emitting region is strongly constrained (as
described in a companion paper) on purely geometrical arguments. The size of
the flare thus derived is much smaller than the size derived from the analysis
of the evolution of the spectral parameters using the quasi-static cooling
formalism, showing that the time evolution of the flare is determined
essentially from the temporal profile of the heating, with the intrinsic decay
of the flaring loop having little relevance. The very high signal-to-noise of
the individual spectra strongly constrains some of the derived physical
parameters. In particular, very significant evidence for a three-fold increase
in coronal abundance and for a large increase in absorbing column density
during the initial phases of the flare evolution is present.
