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 of 189 candidates comprises high signal-to-noise clusters, from 6 to 29. The ESZ clusters are mostly at moderate redshifts (86% with z below 0.3) and span over a decade in mass, up to the rarest and most massive clusters with masses above 10^15 Msol. We discuss the X-ray properties of this sample derived using both a dedicated XMM-Newton follow-up for validation of Planck cluster candidates as well as the analysis of 62 nearby galaxy clusters (z <0.5) present in the XMM archive.