The 41-year-old Italian singer was to perform in the 1,800-year-old Herod Atticus amphitheater at the foot of the Acropolis. More than 4,500 people will have their tickets refunded, according to the concert’s promoters.

Bocelli, who has been blind since age 12, was scheduled to perform with American conductor Lorin Maazel as part of the Hellenic Festival, a summer-spanning series of cultural performances. They did not say what was wrong with Bocelli.

The Tuscany-born vocalist is known as much for his traditional operatic singing as for his pop ballads. Bocelli’s platinum album, Sacred Arias, currently tops the classical charts. His 1999 album, Sogno, earned a Golden Globe award