Greek workers discovered around 1,000 graves, some filled with ancient treasures, while excavating for a subway system in the historic city of Thessaloniki, the state archaeological authority said Monday.

Some of the graves, which dated from the first century B.C. to the 5th century A.D., contained jewelry, coins and various pieces of art, the Greek archaeological service said in a statement.

Thessaloniki was founded around 315 B.C. and flourished during the Roman and Byzantine eras. Today it is the Mediterranean country's second largest city.

Most of the graves — 886 — were just east of the city center in what was the eastern cemetery during Roman and Byzantine times. Those graves ranged from traces of wooden coffins left in simple holes in the ground, to marble enclosures in five-room family mausoleums.

A separate group of 94 graves were found near the city's train station, in what was once part of the city's western cemetery.

More findings were expected as digging for the Thessaloniki metro continues. Digging started in 2006 and the first 13 stations are expected to be done by the end of 2012. A 10-station extension to the west and east has been announced.