McDonald's has opened a franchise just steps away from the gleaming white marble dome of St. Peter's Basilica, giving indigestion to some cardinals and local business owners.

There was no fanfare for the 30 December opening of the U.S. fast food giant's new venue behind a subdued exterior on the picturesque Borgo Pio, just outside the spiritual home of the world's 1.2 billion Roman Catholics.

When the plan emerged last year, one of its most strident critics was Cardinal Elio Sgreccia, who said McDonald's fare was far removed from Roman gastronomic traditions and not the healthiest of foods.

"The mega sandwich shop on Borgo Pio is a disgrace," Sgreccia told La Repubblica newspaper at the time.

"It would be better to use those spaces to help the needy of the area, spaces for hospitality, shelter and help for those who suffer, as the Holy Father teaches," Sgreccia said.

Despite the holy outrage in some quarters, two nuns were spotted on Tuesday lunchtime going inside the fast food joint.

In a statement, McDonald's emphasised that the new restaurant was in a popular tourist area outside the Vatican, although the building itself is Holy See property.

"As is the case whenever McDonald's operates near historic sites anywhere in Italy, this restaurant has been fully adapted with respect to the historical environment," the company said.

Some local business owners had written to Pope Francis to ask him to keep the chain out, for fear it would upset the artistic, culture and social identity of the neighbourhood.

In the letter, consumer group Codacons and a committee set up to protect Borgo Pio said the area, full of restaurants and shops selling religious articles, was already "saturated" and bringing in more tourists could be a security risk.

But some people who frequent the area welcomed the new arrival, including Raffaella Scarano, an Italian woman who works nearby.

"Anything that is good for the economy of our country is fine by me," she said.

Cities across Italy have been turning up the heat on fast food restaurants. McDonald's filed a $20 million lawsuit against Florence after the mayor of the Renaissance city turned down an application to open one of its restaurants there.