More than 5000 tombs and graves were uncovered, some of them containing exquisite golden wreaths.
"The excavations are the biggest archaeological project of recent years in Greece," Yannis Mylopoulos, chairman of Attiko Metro, the company building the network, said. "The quality and the quantity of the findings is impressive."
Several statues of Aphrodite, the goddess of love and beauty, were found, and she was also depicted in mosaics, with one showing her reclining on a couch in front of Eros.
The discovery of statues of Aphrodite dating from as late as 4AD showed that the city "served as a powerful bastion of the old religions until late antiquity", said Professor Polyxeni Adam-Veleni, of the archaeological museum of Thessaloniki
The city was established in 4BC and became a trading and military hub of the Roman empire and, later, the Byzantine empire. It remained powerful into the Medieval era, with a population of more than 100,000 in the 14th century.