A Roman amphitheatre in London once capable of holding 6,000 people has opened its doors to the public for the first time in more than 1,500 years.

What remains of the stadium, which is believed to date from 70AD, opened on Wednesday after 14 years' excavation and preservation work.

The amphitheatre was discovered by accident in 1988 during excavations to extend the mediaeval Guildhall in the city of London, after being buried for centuries under six metres of mud.

It was the second biggest structure in the city after the Forum

Hedley Swain, Museum of London

Now the excavations have been opened for all-weather viewing, allowing modern-day visitors to enter by a door in the arena walls and go straight into the pit - once the scene of terrifying gladiatorial battles.

The Museum of London's head of early history, Hedley Swain, said: "This was a huge building by the standards of Roman London, but we were not even sure the city had an amphitheatre let alone where it might be.

"We estimate that it could seat 6,000 people at a time when the population of the city was about 24,000.

"It was the second biggest structure in the city after the Forum."

The arena, measuring just 60 metres across at its widest point, was not huge by the standards of Rome itself - the whole amphitheatre, seats and all, would have fitted inside the arena of the Colosseum in Rome.

About one quarter of the arena walls remain

But from its location on Ludgate Hill, in the northwest corner of the Roman city it would have been visible from afar.

Stone walls more than two metres high would have held giant earthworks, onto which was placed seating for the crowds.

Criminals and animals would probably have been forced to fight on the floor of the elliptical arena.

Prisoners

"It is likely that travelling troupes of gladiators did visit this northern outpost of the empire occasionally, but we have no proof," said Mr Swain.

"We also know from other amphitheatres that prisoners were executed in the arenas.

"We found some bear bones, but otherwise [we found] no physical evidence of how the arena was used," he added.

London is now believed to have been one of the largest settlements of the Roman empire outside the Mediterranean.

Mr Swain said the excavations cost �4m, and the subsequent preservation and presentation work a further �1.4m.