Workers renovating a rugby stadium have uncovered a vast complex of tombs beneath Rome that mimics the houses, blocks and streets of a real city, according to officials who have unveiled a series of new finds in the Italian capital.

The culture ministry said medieval pottery shards in the city of the dead, or necropolis, indicated the area may have been inhabited by living people during the Dark Ages having been used for centuries during the Roman period for burials.

It is not yet clear who was buried there, but archaeologists at the partially excavated site believe at least some of them were freed slaves of Greek origin.

"It's a matter of a few weeks to discover what is down there," said Marina Piranomonte, an archaeologist. "But it's something big; it looks like a neighbourhood."

A separate dig in the north of the city has revealed the tomb of the nobleman Marcus Nonius Macrinus, who led Rome's legions in the second century AD.

Experts using writings at the site to link it to Macrinus, one of the closest aides and generals of the emperor Marcus Aurelius during his campaigns against Germanic tribes in northern Europe.

A flood of the river Tiber had covered the mausoleum in mud, which collapsed most of the monument but helped preserve decorations, marble columns and inscriptions.

Other discoveries unveiled by the culture ministry news conference included an underground passage in which the despotic emperor Caligula was murdered by his own guards.

The find was made by archaeologists restoring imperial residences on the Palatine Hill in the heart of ancient Rome. The hill, which is honeycombed with ruins of palaces and villas, yielded frescoes and black and white mosaics in the first century BC home of a patrician, the ministry said.

Separately, experts working in Castel di Guido on the outskirts of Rome had enlarged their dig at a previously known complex of country villas owned by Rome's rich and powerful, uncovering fountains, baths and a cistern, the ministry said.

Archaeologists would keep working at the digs to make them accessible to visitors. Officials planned to build a museum next to Macrinus's tomb that would offer a virtual reconstruction of the site.