Every day hordes of London commuters have passed unknowingly over the bodies of thousands of their predecessors, buried a few metres under the roaring traffic and rumbling trains at Liverpool Street, and which are now being exposed for the first time by the huge Crossrail construction project.

The bodies include those of mentally ill patients from Bethlem, the ancient and notorious asylum from which the word Bedlam entered the English language. Bodies that were never claimed by their families – often those of beaten, starved and exploited inmates – would have ended up in the burial ground alongside rich and poor, old and young, victims of plague and war, from across London.

Jay Carver, lead archaeologist on the Crossrail sites – the largest archaeology project in the UK on the largest infrastructure project in Europe – described the site as exceptionally interesting. "Because of its history, we know that this is one of the most diverse burial grounds in London, a real cross section of its people across two centuries. Bone preservation is excellent in the finds we have already made, and we are expecting many important discoveries when we get into the main phase of the excavation."

The trial trenches have already yielded the first treasure from the 40 archaeology sites along the route of Crossrail's tunnelling: a thumbnail-sized golden coin from Venice, pierced so it could be stitched on as expensive decoration on some costly garment – and likely a bad loss when the thread broke and it fell into the gutter some 400 years ago.

The archaeologists have also found a stretch of a superbly engineered Roman road that probably led to a bridge across the river Walbrook. Builders laid logs and brushwood on the boggy ground before building it up in layers, finishing with gravel and rammed clay still so solid and sound it looks modern. Embedded in the road surface was a human bone, possibly washed out of earlier burials nearby, and another loss that must have caused some cursing: a horse shoe. More Roman finds are confidently expected.

The walled, two-acre burial ground was opened in the mid-17th century by order of the mayor of London. It was the first built away from the city's parish churches and their bursting, grossly overfilled graveyards and was usually known as Bedlam because it was on land formerly occupied by the mental hospital, which had recently moved to Moorfields. It would move again to the present site of the Imperial War Museum, and finally to Bromley in Kent, where it survives today as the Bethlem Royal hospital.

Location of Bethlem sites in London.

From the start, because it had a preaching pulpit but no church, it was associated with dissenters — as Bunhill Fields later became. Carver hopes to find evidence of two particularly interesting characters known to have been buried there: 'Freeborn John' – John Lilburne – a radical campaigner and pamphleteer for the rights of the common man who greatly influenced the Levellers, was imprisoned in the Tower of London, exiled twice and eventually died while on parole from his final jail term.

Robert Lockyer, a soldier in Cromwell's New Model Army who was executed for his involvement in the Bishopsgate Mutiny – when the army defied orders to leave London – was also buried there after a funeral that terrified the authorities, attended by thousands of mourners wearing the Levellers' green ribbons.

The victims of several outbreaks of plague were also buried there; as it filled, there were appeals for more top soil to keep the bodies decently covered, and by the time it closed in 1714 it held a 2m layer solid with corpses. Because the bodies came from all over London, those buried there are unusually diverse socially. This poses a problem for Carver; there are no surviving burial records for the cemetery, and instead names are scattered through thousands of records in the parishes where they lived or died. He hopes to ask the public for help in tracking them down.

Part of the cemetery was excavated in the 1980s by the Museum of London, when the Broadgate Centre was built But while towering office blocks gradually replaced the Victorian townhouses, factories and warehouses – which in turn displaced the warren of poor Georgian buildings – an extensive stretch of the burial ground survived under Liverpool Street itself. The busy road, following the route taken by the Romans almost 2,000 years earlier, kept the site as open ground and preserved the remains from being destroyed by pile driving and foundations.

The remains of several hundred individuals have already been found in the trial pits and the trenches dug to relocate utilities. Carver believes he will find up to 4,000 more when the main excavation starts next year.

• This article was amended on 8 August 2013. An earlier version wrongly referred to the Leveller Robert Lockyer as John Lockyer.