In this paper we present and discuss for the first time a large data set of UBVRI night sky brightness measurements collected at ESO-Paranal from April 2000 to September 2001 .
A total of about 3900 images obtained on 174 different nights with FORS1 were analysed using an automatic algorithm specifically designed for this purpose .
This led to the construction of an unprecedented database that allowed us to study in detail a number of effects such as differential zodiacal light contamination , airmass dependency , daily solar activity and moonlight contribution .
Particular care was devoted to the investigation of short time scale variations and micro-auroral events .
The typical dark time night sky brightness values found for Paranal are similar to those reported for other astronomical dark sites at a similar solar cycle phase .
The zenith-corrected values averaged over the whole period are 22.3 , 22.6 , 21.6 20.9 and 19.7 mag arcsec ^ { -2 } in U,B,V,R and I respectively .
In particular , there is no evidence of light pollution either in the broadband photometry or in the high-airmass spectra we have analysed .
Finally , possible applications for the exposure time calculators are discussed .