Rome's conservative mayor Gianni Alemanno received a letter and a bullet through the mail at his office in the town hall.
The typed letter was said to have been signed 'The Armed Patriotic Groups' (GAP) and was posted to Alemanno with the bullet inside a yellow envelope. The items were being examined by police forensic experts.
Alemanno described the letter as 'elaborate and ideological'. GAP was also the Italian acronym for the World War II partisan resistance formation The Patriotic Action Groups set up in 1943, which carried out acts of sabotage and guerrilla warfare.
A number of threats signed by purported left-wing extremists have been mailed to conservative and centre-left politicians as well as other prominent figures over the past eight months.
Alemanno, Labour Minister Maurizio Sacconi and other public figures including the head of Italy's main private employers' association Emma Marcegaglia received mailed threats earlier this year.
The threats were posted in January to AKI in Rome and signed 'For Communism, the Red Brigades, and the Galesi Centre for an Anti-imperialist Armed Movement'.
Mario Galesi was a new Red Brigades member and murder suspect killed in a shoot-out with police aboard a train in March, 2003.