The Arch of Constantine, a giant ancient Roman arch next to the Colosseum, was damaged after a violent storm hit Rome, a Reuters reporter saw on Tuesday.
The triumphal arch was built in the 4th century AD to celebrate the victory of Constantine - the first Roman emperor to convert to Christianity - over his rival, Maxentius.
It is about 25 metres (82 feet) high and is located in the same pedestrian area where the Colosseum stands, a major tourist hotspot.
"A lightning strike hit the arch right here and then hit the corner and we saw this fly off," a tourist told Reuters, pointing to a large block of stone that had fallen to the ground.
Reuters video images showed other blocks of stone and rubble lying around the monument, and Colosseum Archaeological Park staff working to collect them.
The accident was said to have taken place during a heavy thunderstorm that felled trees and branches and flooded several streets of the Italian capital.
A spokesperson for the park was not immediately available for comment.
The Civil Protection agency said 60 millimetres (2.36 inches) of rain fell on central Rome in less than one hour, about as much as would normally fall in a month during autumn.
