  The compact radio source Sgr A*, believed to be the counterpart of the
massive black hole at the Galactic nucleus, was observed to undergo rapid and
intense flaring activity in X-rays with Chandra in October 2000. We report here
the detection with XMM-Newton EPIC cameras of the early phase of a similar
X-ray flare from this source, which occurred on 2001 September 4. The source
2-10 keV luminosity increased by a factor ~ 20 to reach a level of 4 10$^{34}$
erg s$^{-1}$ in a time interval of about 900 s, just before the end of the
observation. The data indicate that the source spectrum was hard during the
flare and can be described by simple power law of slope ~ 0.7. This XMM-Newton
observation confirms the results obtained by Chandra, suggests that, in Sgr A*,
rapid and intense X-ray flaring is not a rare event and therefore sets some
constraints on the emission mechanism models proposed for this source.
