In a statement to Reuters, which first reported on the accident, the Colosseum Archaeological Park on Tuesday confirmed that the monument was hit by lightning.
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 high and 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 fallen to the ground.
Reuters video images showed other blocks of stone and rubble lying around the monument, and archaeological park staff working to collect them.
"All fragments were recovered and secured. Damage assessments have already begun and the analyses will continue tomorrow morning," the archaeological park said.
The accident took place during a heavy thunderstorm that felled trees and branches and flooded several streets of the Italian capital.
The Civil Protection agency said 60 millimetres of rain fell on central Rome in less than one hour, about as much as would normally fall in a month during autumn.
The freak weather was a so-called "downburst", Rome Mayor Roberto Gualtieri said, referring to the kind of severe storm, featuring powerful downward winds, believed to have also caused last month's sinking of British tech tycoon Mike Lynch's yacht off Sicily. Seven people died, including Lynch.
"The event that hit Rome is truly unprecedented, because it was so powerful and concentrated in a very short time and in some areas of the city, starting from the historic centre," Gualtieri said in a statement.
