  We present FORS2 (attached to ESO's Very Large Telescope) observations of the
exoplanet WASP-17b during its primary transit, for the purpose of differential
spectrophotometry analysis. We use the instrument in its Mask eXchange Unit
(MXU) mode to simultaneously obtain low resolution spectra of the planet
hosting star, as well as several reference stars in the field of view. The
integration of these spectra within broadband and smaller 100$\AA$ bins
provides us with 'white' and spectrophotometric light curves, from 5700 to
8000$\AA$. Through modelling the white light curve, we obtain refined bulk and
transit parameters of the planet, as well as wavelength-dependent variations of
the planetary radius from smaller spectral bins through which the transmission
spectrum is obtained. The inference of transit parameters, as well as the noise
statistics, is performed using a Gaussian Process model. We achieve a typical
precision in the transit depth of a few hundred parts per million from various
transit light curves. From the transmission spectra we rule out a flat spectrum
at >3$\sigma$ and detect marginal presence of the pressure-broadened sodium
wings. Furthermore, we detect the wing of the potassium absorption line in the
upper atmosphere of the planet with 3$\sigma$ confidence, both facts pointing
to a relatively shallow temperature gradient of the atmosphere. These
conclusions are mostly consistent with previous studies of this exo-atmosphere,
although previous potassium measurements have been inconclusive.
