The coins date back to the end of the Roman Empire in the 5th century and were found in a kind of stone urn in the Cressoni theatre basement, not far from the site of the ancient city of Novum Comum.
According to Italian media, the coins could be worth millions of euros.
“We do not yet know in detail the historical and cultural significance of this discovery but this area is a real treasure for our archeology,” said Culture Minister Alberto Bonisoli on Facebook.
The theatre, which was inaugurated in 1870 and later became a cinema before closing in 1997, was due to be demolished to allow the construction of a luxury residence.
Authorities now plan to suspend work at the site to allow further excavations, according to local media.