We present the first all-sky sample of galaxy clusters detected via the Sunyaev-Zel'dovich effect from the Planck survey. This Early SZ (ESZ) sample of 189 candidates comprises high signal-to-noise clusters. It is highly reliable (purity above 95%). Planck provides the first measured SZ signal for about 80% of the 169 ESZ known clusters and releases 30 new cluster candidates. We have engaged an ambitious validation program with XMM-Newton, observing twenty-five new detected SZ candidates using snapshot exposures. A total of 21 are confirmed as extended X-ray sources. Most are single clusters with highly irregular and disturbed morphologies, the rest being either double or unexpected triple systems. For most of these sources we derived redshift estimates and a first estimate of their X-ray properties. Our results suggest that Planck may have started to reveal a non-negligible population of massive dynamically perturbed objects that is under-represented in X-ray surveys.