  The branching ratio (BR) for the decay K^+ \to \pi^+\nu\bar{\nu} is a
sensitive probe for new physics. The NA62 experiment at the CERN SPS will
measure this BR to within about 10%. To reject the background from dominant
kaon decays with final state photons, the large-angle photon vetoes (LAVs) must
detect photons of energy as low as 200 MeV with an inefficiency of less than
10^{-4}. The LAV detectors make use of lead glass blocks recycled from the OPAL
electromagnetic calorimeter barrel. We describe the mechanical design and
challenges faced during construction, the characterization of the lead glass
blocks and solutions adopted for monitoring their performance, and the
development of front-end electronics to allow simultaneous time and energy
measurements over an extended dynamic range using the time over-threshold
technique. Our results are based on test-beam data and are reproduced by a
detailed Monte Carlo simulation that includes the readout chain.
