The reporter, Ahmad Rafat, says he was forced to surrender his accreditation to the World Food Security Summit, which is being held at the UN Food and Agriculture Organization's (FAO) headquarters.
The Iranian-born Rafat is based in Rome and is a regular correspondent for Radio Farda, a joint venture between RFE/RL and Voice of America that broadcasts to Iran.
He said he was told by Italian guards that he had been declared persona non grata and that a security officer had "made it understood that this was because of pressure [by] a foreign delegation."
"I think my reports about the protests of political [groups] over Ahmadinejad's trip to Italy and the fact that no political force or official is willing to meet with him -- the reports which I did for Radio Farda in the past days and also for the news agency [Adnkronos] for which I am the editor in chief here in Italy -- are the only issues that could have caused this reaction," Rafat told Radio Farda. "I haven't done anything but write."