  We present optical to very-high energy (VHE) gamma-ray observations of Mrk
421 between 2008 May 24 and June 23. A high-energy (HE) gamma-ray signal was
detected by AGILE-GRID during June 9-15, brighter than the average flux
observed by EGRET in Mrk 421 by a factor of approx. 1.5. In 20-60 keV X-rays, a
large-amplitude 5-day flare (June 9-15) was resolved with a maximum flux of
approx. 55 mCrab. SuperAGILE, RXTE/ASM and Swift/BAT data show a clearly
correlated flaring structure between soft and hard X-rays, with a high
flux/amplitude variability in hard X-rays. Hints of the same flaring behavior
is also detected in the simultaneously recorded GASP-WEBT optical data. A
target of opportunity observation by Swift near the flare maximum on June 12-13
revealed the highest 2-10 keV flux ever observed (>100 mCrab) and a peak
synchrotron energy of approx. 3 keV, a large shift from typical values of 0.5-1
keV. Observations at VHE (E>200 GeV) gamma-rays during June 6-8 show the source
flux peaking in a bright state, well correlated with the simultaneous peak in
the X-rays. The gamma-ray flare can be interpreted within the framework of the
Synchrotron Self Compton model in terms of a rapid acceleration of leptons in
the jet.
