Future detection of extensive air showers (EAS) produced by ultra high energy cosmic particles by means of space-based fluorescence telescopes will open a new window on the universe and allow cosmic ray and neutrino astronomy at a level that is virtually impossible for ground based detectors. In the context of the extreme universe space observatory (EUSO) project, an end-to-end simulation of EAS observation with a spatial detector has been designed (EUSO simulation and analysis framework, ESAF). This paper describes the detailed Monte-Carlo developed to simulate all the physical processes involved in the fluorescence detection technique, from the EAS development to the instrument response. Particular emphasis is given to modeling the light propagation in the atmosphere and the effect of clouds. The simulation is used to assess the performances of EAS spatial detection. Main results on energy threshold and resolution, direction resolution and X max determination are reported. Results are based on EUSO telescope design, but are also extended to larger and more sensitive detectors.