  We describe the all-sky Planck catalogue of clusters and cluster candidates
derived from Sunyaev--Zeldovich (SZ) effect detections using the first 15.5
months of Planck satellite observations. The catalogue contains 1227 entries,
making it over six times the size of the Planck Early SZ (ESZ) sample and the
largest SZ-selected catalogue to date. It contains 861 confirmed clusters, of
which 178 have been confirmed as clusters, mostly through follow-up
observations, and a further 683 are previously-known clusters. The remaining
366 have the status of cluster candidates, and we divide them into three
classes according to the quality of evidence that they are likely to be true
clusters. The Planck SZ catalogue is the deepest all-sky cluster catalogue,
with redshifts up to about one, and spans the broadest cluster mass range from
(0.1 to 1.6) 10^{15}Msun. Confirmation of cluster candidates through comparison
with existing surveys or cluster catalogues is extensively described, as is the
statistical characterization of the catalogue in terms of completeness and
statistical reliability. The outputs of the validation process are provided as
additional information. This gives, in particular, an ensemble of 813 cluster
redshifts, and for all these Planck clusters we also include a mass estimated
from a newly-proposed SZ-mass proxy. A refined measure of the SZ Compton
parameter for the clusters with X-ray counter-parts is provided, as is an X-ray
flux for all the Planck clusters not previously detected in X-ray surveys.
