The wall, part of a 16th century restoration, crumbled into a pile of bricks Thursday evening after water infiltrated the section, said Paola Virgili, an official in charge of the wall's restoration.
The Aurelian Wall named after the third century emperor who built it to defend the city against the first barbarian onslaughts surrounds Rome with more than 17 kilometers (11 miles) of fortifications, towers and gates.
Experts had previously determined that the entire wall section in the area, a 350-meter (1,100-foot) stretch in the north of the capital, was in danger of collapsing and they had planned to start restoring it Monday.
"It came down before we could even cordon it off," Virgili said. "The problem is that these walls have a certain age and they are vulnerable to water infiltration."
Virgili said workers would put up emergency buttresses to shore up the collapsed section, while restoration on the entire endangered stretch would begin in a few months. It was not yet clear how much the works would cost, but Virgili said her budget stood at Ђ2 million (US$2.88 million).
Thursday's collapse was not the first.
In 2001, a 30-meter (100-foot stretch) in the south of Rome collapsed and it took years and millions of euros to restore it. The city regularly allocates money for the monument, but the amounts are usually only enough to renovate the most fragile sections.
Virgili said a full restoration of the entire wall would cost at least Ђ100 million (US$144 million).