  After 25 years of quiescence, the microquasar V404 Cyg entered a new period
of activity in June 2015. This X-ray source is known to undergo extremely
bright and variable outbursts seen at all wavelengths. It is therefore an
object of prime interest to understand the accretion-ejection connections.
These can, however, only be probed through simultaneous observations at several
wavelengths. We made use of the INTEGRAL instruments to obtain long, almost
uninterrupted observations from 2015 June 20$^{\mathrm{th}}$, 15:50 UTC to June
25$^{\mathrm{th}}$, 4:05 UTC, from the optical V-band, up to the soft
$\gamma$-rays. V404 Cyg was extremely variable in all bands, with the detection
of 18 flares with fluxes exceeding 6 Crab (20--40 keV) within 3 days. The flare
recurrence can be as short as $\sim$ 20~min from peak to peak. A
model-independent analysis shows that the $>$6 Crab flares have a hard
spectrum. A simple 10--400 keV spectral analysis of the off-flare and flare
periods shows that the variation in intensity is likely to be due to variations
of a cut-off power law component only. The optical flares seem to be at least
of two different types: one occurring in simultaneity with the X-ray flares,
the other showing a delay greater than 10 min. The former could be associated
with X-ray reprocessing by either an accretion disk or the companion star. We
suggest that the latter are associated with plasma ejections that have also
been seen in radio.
