The iconic metal bridge, which was opened 1863, went up in flames just before midnight.
The fire took place on the eve before Romans took to the polls to elect a new mayor in local elections, in which the main issue has been the general decay of infrastructure and public services in the Italian capital.
The city’s outgoing mayor, Virginia Raggi, arrived at the scene, saying: “It is heartbreaking to see a piece of history reduced to this.”
“The important thing is that nobody was injured,” she added.
An investigation is underway into the cause of the fire, which may have originated from makeshift shacks under the 158-year-old bridge, and involved electric and gas pipelines.
Italian newspaper Corriere della Sera reported that in addition to a large amount of rubbish under the bridge, gas cylinders were found, used by homeless people living there.
The fire led to surrounding areas being left without gas, electricity and water for several hours.
The bridge, used for busy road traffic between the two densely populated Ostiense and Portuense neighbourhoods, was closed indefinitely pending checks on its safety.
