  The Herschel-ATLAS is a survey of 550 square degrees with the Herschel Space
Observatory in five far--infrared and submillimetre bands. The first data for
the survey, observations of a field 4x4 sq. degrees in size, were taken during
the Science Demonstration Phase, and reach a 5 sigma noise level of 33 mJy/beam
at 250 microns. This paper describes the source extraction methods used to
create the corresponding Science Demonstration Phase catalogue, which contains
6876 sources, selected at 250 microns, within ~14 sq. degrees. SPIRE sources
are extracted using a new method specifically developed for Herschel data; PACS
counterparts of these sources are identified using circular apertures placed at
the SPIRE positions. Aperture flux densities are measured for sources
identified as extended after matching to optical wavelengths. The reliability
of this catalogue is also discussed, using full simulated maps at the three
SPIRE bands. These show that a significant number of sources at 350 and 500
microns have undergone flux density enhancements of up to a factor of ~2, due
mainly to source confusion. Correction factors are determined for these
effects. The SDP dataset and corresponding catalogue will be available from
http://www.h-atlas.org/.
