Context : HD 189733 is an exoplanetary system consisting of a transiting hot Jupiter and an active K2V-type main sequence star .
Rich manifestations of a stellar activity , like photometric spots or chromospheric flares were repeatedly observed in this system in optical , UV and X-rays .

Aims : We aim to use VLT/UVES high resolution ( R = 60 000 ) echelle spectra to study a stellar flare .

Methods : We have performed simultaneous analyses of the temporal evolution in several chromospheric stellar lines , namely , the Ca II H & K lines ( 3933 , 3968 Å ) , H \alpha ( 6563 Å ) , H \beta ( 4861 Å ) , H \gamma ( 4341 Å ) , H \delta ( 4102 Å ) , H \epsilon ( 3970 Å ) , the Ca II infrared triplet line ( 8498 , 8542 and 8662 Å ) , and He I D3 ( 5875.6 Å ) .
Observations were carried out with a time resolution of approximately 1 min for a duration of four hours , including a complete planetary transit .

Results : We determine the energy released during the flare in all studied chromospheric lines combined to be about 8.7 \times 10 ^ { 31 } 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 & 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 & K lines during the flare , we estimate the soft X-ray flux emission to be 7 \times 10 ^ { 30 } erg .

Conclusions : 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 .