Arts centre discovered under one of Rome's busiest roundabouts was built in AD123 and could seat 900 people

Archaeologists who have completed the excavation of a 900-seat arts centre under one of Rome's busiest roundabouts are calling it the most important Roman discovery in 80 years.

The centre, built by the emperor Hadrian in AD123, offered three massive halls where Roman nobles flocked to hear poetry, speeches and philosophy tracts while reclining on terraced marble seating.

With the dig now completed, the terracing and the hulking brick walls of the complex, as well as stretches of the elegant grey and yellow marble flooring, are newly visible at bottom of a 5.5 metre (18ft) hole in Piazza Venezia, where police officers wearing white gloves direct chaotic traffic like orchestra conductors and where Mussolini harangued thousands of followers from his balcony.

"Hadrian's auditorium is the biggest find in Rome since the Forum was uncovered in the 1920s," said Rossella Rea, the archaeologist running the dig.

The excavations, which are now due to open to the public, are next to a taxi rank and squeezed between a baroque church and the Vittoriano, an imposing monument to Italy's defunct monarchy, which is nicknamed the Typewriter by locals.

The complex was only unearthed thanks to excavations to build a new underground railway line which will cross the heart of Rome. "We don't have funds for these kind of digs so this has come to light thanks to the new line," said Rea.

Archaeologists keeping a careful eye on what gets dug up have proved to be a mixed blessing for railway engineers, who have had to scrap plans for two stations in the heart of the centre of Rome when it was discovered their exits to the surface cut straight through Roman remains.

With the discovery of Hadrian's complex at Piazza Venezia, the line risked losing its last stop in the centre and being forced to run into the heart of Rome from the suburbs and straight out the other side without stopping. But Rea said the station and the ruins could coexist.

"I believe we can run one of the exits from the station along the original corridor of the complex where Romans entered the halls," she said.

The site sheds new light on Hadrian's love of poetry – he wrote his own verse in Latin and Greek – and his taste for bold architecture – an 11-metre-high (36ft) arched ceiling once towered over the poets in the central hall.

Today the performing space is riddled with pits dug for fires, revealing how after three centuries of celebrating the arts, the halls fell into disrepair with the collapse of the Roman empire and were used for smelting ingots.

At the centre of the main hall, like a prop from a disaster movie, is a massive, nine-by-five-metre chunk of the monumental roof which came crashing down during an earthquake in 848 after standing for seven centuries.

Following the quake, the halls were gradually covered over until a hospital built on top in the 16th century dug down for cellar space. "We found pots lobbed down a well after the patients using them died," said Rea. "We could date them because the designs on the glaze were the same we see on implements in Caravaggio paintings."