Luxury watchmaker Rolex took out a page in several Italian newspapers to distance its brand from a rioter photographed with a luxury watch on her wrist as she spray-painted buildings in Milan, during violent protests against the opening of the Universal Exhibition in the city.

The picture of the black clad anarchist with a taste for expensive timepieces was taken amid the chaos that engulfed Italy's business capital on May Day, when baton-wielding demonstrators wearing motorcycle helmets pelted police with stones and set vehicles and shops ablaze.

The image went viral online and was seized upon by the government to dismiss protesters as spoiled thugs moved by no ideology rather than that of senseless violence.

Interior Minister Angelino Alfano described them as "masked crooks and Rolex-wearing preppy boys", while Prime Minister Matteo Renzi praised Milan residents who took to the streets to clean up the city after the violence saying: "While those with Rolexes were out smashing shop windows, they [locals] took it on themselves to tidy up."

The Swiss manufacturer has now demanded both officials rectify their statements on the grounds that from the photo it is not clear whether or not the female rioter was actually wearing a real Rolex - the cheapest model approximately £5,000 ($7,500, €6,700)

In an open letter to Renzi and Alfano published as an ad in several newspapers, the company implied it was unacceptable for its expensive products to become infamous as a rioters' watch.

"Due to the [poor] quality of photos and videos broadcast by media, it is highly unlikely that the watch the hooligans wore can reliably be identified as a Rolex (and even more so as an authentic Rolex)", the letter signed by Rolex Italy managing director Gianpaolo Marini read.

"I have to express my regret and disappointment at the incidental association your words made between smashing shop windows and wearing a Rolex."

The government didn't immediately comment on the letter, which was read by some as a PR stunt.

Up to 1,500 radicals, known in Italy as Black Blocs, wreaked havoc in Milan last week clashing with police on the side-lines of the opening of Expo Milano 2015.

The violence overshadowed a largely peaceful protest against the food-themed world fair, which demonstrators believe will not benefit Italy but instead the corporations collaborating with the event. Protesters also claimed that thousands of people are working for free as volunteers at the international event.

The Expo inauguration was nevertheless hailed as a success by organisers, with an estimated 650,000 visitors in the first three days of exhibition.

Some 145 countries are taking part in the event that has the slogan "Feeding the Planet, Energy for Life" as central theme.