The 2,000-year old arena, where gladiators fought bloody battles for the entertainment of crowds, originally looked far different from the stone ring that has become one of the symbols of Rome.

Working in a passage closed to the public for decades, restorers scraped off years of limescale and black pollution from car exhaust to discover remains of the frescoes, their vivid red, blue, green and white colours still visible.

"This is a beautiful archaeological surprise," Mariarosaria Barbera, Rome's archaeological superintendent told Reuters on Friday. "Even in a monument as well known as this one, studied all over the world, there are still new things to discover."

The team also discovered ancient sketches by spectators who painted crowns and palm trees, symbols of victory celebrating the success of gladiators they supported.

The Latin word "VIND", referring to victory or revenge, was also found.

Restorers discovered the frescoes in a passage leading to the highest level of seating, a wooden gallery reserved for the lowest classes and furthest from the action in the arena.

Senators had seats on the first floor, while the emperor and Vestal Virgin priestesses had special boxes with the best views.

Blue pigment was a costly luxury at the time, so its use in a corridor leading to the cheap seats indicated the rest of the stadium's interior may have been intricately decorated too, Barbera said.

The frescoes likely date from after 217 AD, when a fire destroyed the wooden gallery that topped the Colosseum.

NEW RESTORATION

Known in Roman times as the Flavian Amphitheatre, the Colosseum has suffered heavily in two millennia, not least from pollution from a road ringing the monument just steps away.

The exterior of the building visited by millions of tourists each year was originally covered in gleaming white marble that was removed over the centuries to be used elsewhere.

The frescoes were discovered during the monument's first comprehensive restoration in 73 years, a 25 million euro project to clean the entire building by 2015.

After the restoration, some 25 percent more of the Colosseum will be open to visitors, particularly the underground network of tunnels, storerooms and cages.

The project is funded by luxury shoemaker Tod's (TOD.MI), one of several private investors to step in to help the indebted Italian government maintain the country's wealth of historic monuments.

Restorers have cleaned only a small part of the monument so far, and hope to reveal the detail of what the frescoes depict underneath marks left by centuries of visitors.

Written in a modern script, the name "Luigi" was scratched into a well-preserved red section of fresco. Nearby was scrawled the date "1620", and "J. Milber from Strasbourg, 1902". (Reporting by Naomi O'Leary)

• A Permanent Economic Emergency