  We present the first all-sky sample of galaxy clusters detected blindly by
the Planck satellite through the Sunyaev-Zeldovich (SZ) effect from its six
highest frequencies. This early SZ (ESZ) sample is comprised of 189 candidates,
which have a high signal-to-noise ratio ranging from 6 to 29. Its high
reliability (purity above 95%) is further ensured by an extensive validation
process based on Planck internal quality assessments and by external
cross-identification and follow-up observations. Planck provides the first
measured SZ signal for about 80% of the 169 previously-known ESZ clusters.
Planck furthermore releases 30 new cluster candidates, amongst which 20 meet
the ESZ signal-to-noise selection criterion. At the submission date, twelve of
the 20 ESZ candidates were confirmed as new clusters, with eleven confirmed
using XMM-Newton snapshot observations, most of them with disturbed
morphologies and low luminosities. The ESZ clusters are mostly at moderate
redshifts (86% with z below 0.3) and span more than a decade in mass, up to the
rarest and most massive clusters with masses above 10^15 Msol.
