  We present the X-ray pipeline developed for the purpose of the cluster search
in the XMM-LSS survey. It is based on a two-stage procedure via a dedicated
handling of the Poisson nature of the signal: (1) source detection on
multi-resolution wavelet filtered images; (2) source analysis by means of a
maximum likelihood fit to the photon images. The source detection efficiency
and characterisation are studied through extensive Monte-Carlo simulations.
This led us to define two samples of extended sources: the C1 class that is
uncontaminated, and the less restrictive C2 class that allows for 50%
contamination. The resulting predicted selection function is presented and the
comparison to the current XMM-LSS confirmed cluster sample shows very good
agreement. We arrive at average predicted source densities of about 7 C1 and 12
C2 per deg2, which is higher than any available wide field X-ray survey. We
finally notice a substantial deviation of the predicted redshift distribution
for our samples from the one obtained using the usual assumption of a flux
limited sample.
