The OPERA VETO system

Abstract OPERA is an underground experiment to search for ν τ appearance from a pure ν μ beam produced at CERN. To flag the events due to the neutrino interactions with the rock surrounding the OPERA detector, a large VETO system, based on the use of Glass Resistive Plate Chambers (GRPCs) has been realized. We describe the VETO system, the GRPCs, the tests performed on the chambers before their installation. Presently, the VETO is in phase of commissioning. The first results of its underground operation are also presented.