The first phase of a multi-million-euro makeover of Rome's Colosseum was completed Friday with Prime Minister Matteo Renzi pledging cash would be made available to spruce up other crumbling historic sites.

In a project largely funded by fashion and shoewear group Tod's, the amphitheatre where gladiators once jousted with lions has been water-sprayed to remove centuries of encrusted dirt and grime.

Works to strengthen the arched structures of the northern and southern facades and replace metal gates and barriers in the ground level arches have also been completed.

Tod's, whose billionaire owner Diego Della Valle reportedly put up 25 million euros for the works, said it was proud to have been part of the restoration of "a true historical symbol of Italy."

The Colosseum is the latest in a string of famous Italian monuments to have been renovated with funds from private donors, often from the luxury sector.

Roman fashion house Fendi paid for a 16-month clean-up of the Trevi fountain which has been acclaimed by visitors. And upmarket jeweller Bulgari is behind the ongoing renovation of the Spanish steps, also located in the capital's historic centre.

But across the country there are many historic sites which have fallen into disrepair due to a lack of funds, most notably the ancient archaelogical site of Pompeii.

Renzi vowed that would not continue. "We have to stop the arguments over Italy's cultural heritage because it is not only the thing we can be most proud of and a major part of our identity, but it also has enormous potential," he said.

"The time of complaining there is no money for culture is over. Public and private, the resources are there."

Renzi's government has promised 18 million euros ($20 million) for a second phase of renovation of the Colosseum which will involve rebuilding the arena floor and make it capable of hosting concerts and other cultural events, including re-enactments of some of the kind of shows the ancient Romans enjoyed.

The floor was removed by excavators in the late 19th century while the bits of the exterior structure that are missing were mostly removed for other construction projects in the city, including the underground.

There are also plans for a new visitor centre and the renovation of the underground vaults where wild animals and prisoners destined for public execution were kept ahead of their appearances before the Roman crowds.

Completed in 80 AD, the Colosseum was the biggest amphitheatre built during the Roman empire.

It stands 48.5 metres (159 feet) high and was capable of hosting 80,000 spectators. It now welcomes over six million visitors a year.