A huge fire on Saturday night severely damaged Rome's famed "Iron Bridge", with parts of the 19th-century structure plunging into the Tiber River.

The bridge, which was opened in 1863 and whose formal name is Ponte dell' Industria (Industry Bridge), connects the densely populated Ostiense and Portuense neighbourhoods.

Pope Pius IX attended the 1863 inauguration of the bridge, one of the last major construction works in Rome in the waning years of the papal state controlling the city, which would soon become the capital of unified Italy.

The fire broke out on the eve of elections for the next mayor, in which the main issue has been the general decay of infrastructure and public services in the Italian capital.

Romans call the 131-metre long bridge "Ponte di Ferro" (Iron Bridge), since most of Rome's other bridges are made of stone.

Officials said there were no injuries from the fire, which was visible from afar as flames shot into the night sky.

Media reports said it may have been started by a short circuit in shacks below the bridge. Firefighters said the blaze, which was put out during the night, was fuelled by a damaged gas pipe.

Surrounding areas were left without electricity for several hours.

The bridge, used for busy road traffic between the two neighbourhoods, was closed indefinitely pending checks on its safety.

A plaque at the bridge pays tribute to 10 women who were executed on it in 1944 by German SS troops occupying Rome during the latter years of World War II. The women were punished for having occupied a bakery to feed their families in the city where war made food scarce.

Additional input from The Associated Press.