Planck is a European Space Agency (ESA) mission, with significant contributions from the U.S. National Aeronautics and Space Agency (NASA). It is the third generation of space-based cosmic microwave background experiments, after the Cosmic Background Explorer (COBE) and the Wilkinson Microwave Anisotropy Probe (WMAP). Planck was launched on 14 May 2009 on an Ariane 5 rocket from Kourou, French Guiana. Following a cruise to the Earth-Sun L2 Lagrange point, cooling and in orbit checkout, Planck initiated the First Light Survey on 13 August 2009. Since then, Planck has been continuously measuring the intensity of the sky over a range of frequencies from 30 to 857GHz (wavelengths of 1cm to 350{mu}m) with spatial resolutions ranging from about 33' to 5' respectively. The Low Frequency Instrument (LFI) on Planck provides temperature and polarization information using radiometers which operate between 30 and 70GHz. The High Frequency Instrument (HFI) uses pairs of polarization-sensitive bolometers at each of four frequencies between 100 and 353GHz but does not measure polarization information in the two upper HFI bands at 545 and 857GHz. The lowest frequencies overlap with WMAP, and the highest frequencies extend far into the submillimeter in order to improve separation between Galactic foregrounds and the cosmic microwave background (CMB). By extending to wavelengths longer than those at which the Infrared Astronomical Satellite (IRAS) operated, Planck is providing an unprecedented window into dust emission at far-infrared and submillimeter wavelengths. The Planck Early Release Compact Source Catalogue (ERCSC) is a list of all high reliability sources, both Galactic and extragalactic, derived from the first sky coverage. The data that went into this early release comprise all observations undertaken between 13 August 2009 and 6 June 2010, corresponding to Planck operational days 91-389. Since the Planck scan strategy results in the entire sky being observed every 6 months, the data considered in this release correspond to more than the first sky coverage. The source lists have reliability goals of >90% across the entire sky and >95% at high Galactic latitude. The goals on photometric accuracy are 30% while the positional accuracy goal translates to a positional root mean square (RMS) uncertainty that is less than 1/5 of the beam full width at half maximum (FWHM). Detailed explanations about the mission and the catalogs included here can be found in the "Explanatory supplement" (file "ercsc4_3.pdf"). Skymaps of the sources can be found in the "skymaps" subdirectory; postage stamps of the sources in the ECC (Early Cold Cores) catalog and in the different filters are located in the "stamps" subdirectory. The "Byte-by-byte Description" below contain column names standardized according to the conventions used at CDS; the original column names, as defined in the FITS files, are listed, enclosed within parentheses, at the end of the explanations.