The Rev. Al Sharpton was an F.B.I. informer who had extensive dealings with Mafia figures in New York in the 1980s and secretly recorded their conversations using a briefcase modified with hidden electronic equipment, a report said on Monday.

The report, on the website The Smoking Gun, said that Mr. Sharpton had worked with a joint task force overseen by the Federal Bureau of Investigation and the Police Department. The Smoking Gun posted dozens of pages of documents that appeared to provide details of Mr. Sharpton’s dealings with mob figures. In what The Smoking Gun described as secret affidavits that were posted on the site, he was referred to as CI-7, short for Confidential Informant No. 7.

Mr. Sharpton played down his involvement with Mafia figures, although it has long been known that he worked with the F.B.I. in the 1980s in an investigation of the boxing promoter Don King. “The claim is I helped get the mob, not that I was in the mob,” he said in a telephone interview on Monday. “I was never told I was an informant.”

He took issue with the documents The Smoking Gun had posted, which it said included court documents and F.B.I. memos released in response to Freedom of Information Act requests. The report said, for example, that one F.B.I. affidavit seeking court permission for several wiretaps was rewritten. The original version, the report said, was so detailed that Mr. Sharpton could have been identified and his life endangered.