  (Abridged) A brief description of the methodology of construction, contents
and usage of the Planck Early Release Compact Source Catalogue (ERCSC),
including the Early Cold Cores (ECC) and the Early Sunyaev-Zeldovich (ESZ)
cluster catalogue is provided. The catalogue is based on data that consist of
mapping the entire sky once and 60% of the sky a second time by Planck, thereby
comprising the first high sensitivity radio/submillimetre observations of the
entire sky. A Monte-Carlo algorithm based on the injection and extraction of
artificial sources into the Planck maps was implemented to select reliable
sources among all extracted candidates such that the cumulative reliability of
the catalogue is >=90%. The 10sigma photometric flux density limit of the
catalogue at |b|>30 deg is 0.49, 1.0, 0.67, 0.5, 0.33, 0.28, 0.25, 0.47 and
0.82 Jy at each of the nine frequencies between 30 and 857 GHz. Sources which
are up to a factor of ~2 fainter than this limit, and which are present in
"clean" regions of the Galaxy where the sky background due to emission from the
interstellar medium is low, are included in the ERCSC if they meet the high
reliability criterion. The Planck ERCSC sources have known associations to
stars with dust shells, stellar cores, radio galaxies, blazars, infrared
luminous galaxies and Galactic interstellar medium features. A significant
fraction of unclassified sources are also present in the catalogs. In addition,
two early release catalogs that contain 915 cold molecular cloud core
candidates and 189 SZ cluster candidates that have been generated using
multi-frequency algorithms are presented. The entire source list, with more
than 15000 unique sources, is ripe for follow-up characterisation with
Herschel, ATCA, VLA, SOFIA, ALMA and other ground-based observing facilities.
