  HD 189733 is an exoplanetary system consisting of a transiting hot Jupiter
and an active K2V-type main sequence star. We aim to use VLT/UVES high
resolution echelle spectra to study a stellar flare. We have performed
simultaneous analyses of the temporal evolution in several chromospheric
stellar lines, namely, the Ca II H and K lines, Halpha, Hbeta, Hgamma, Hdelta,
Hepsilon, the Ca II infrared triplet line, and He I D3. Observations were
carried out with a time resolution of approximately 1 min for a duration of
four hours, including a complete planetary transit. We determine the energy
released during the flare in all studied chromospheric lines combined to be
about 8.7e31 erg, which puts this event at the upper end of flare energies
observed on the Sun. Our analysis does not reveal any significant delay of the
flare peak observed in the Balmer and Ca II H and K lines, although we find a
clear difference in the temporal evolution of these lines. The He I D3 shows
additional absorption possibly related to the flare event. Based on the flux
released in Ca II H and K lines during the flare, we estimate the soft X-ray
flux emission to be 7e30 erg. The observed flare can be ranked as a moderate
flare on a K-type star and confirms a rather high activity level of HD 189733
host star. The cores of the studied chromospheric lines demonstrate the same
behavior and let us study the flare evolution. We demonstrate that the activity
of an exoplanet host star can play an important role in the detection of
exoplanet atmospheres, since these are frequently discovered as an additional
absorption in the line cores. A possible star-planet interaction responsible
for a flare occurrence during a transit can neither be confirmed nor ruled out.
