A reputed Mafia "soldier" who was indicted with Labor Secretary Raymond J. Donovan charged today that the FBI tape recordings underlying the case were obtained by "blatantly lawless" tactics "unworthy of acceptance in a court of law."

William Masselli, a reputed member of the Genovese crime family, maintained through his lawyer that he only recently learned of the government "deceptions" as a result of the discovery process this fall in the Donovan case.

Masselli's lawyer, John Nicholas Iannuzzi, said the tapes should be suppressed.

The FBI inquiry was reported to have turned up extensive indications of political corruption after it was launched in January 1979 with court-authorized electronic surveillance of Masselli's warehouse-headquarters in the South Bronx.

In an affidavit submitted in court today, Masselli said he was stopped by "approximately 15 FBI agents in various cars and vehicles" while driving to work at 5 a.m. one day around February 1980.

Masselli said one agent played portions of the tape recordings for him and spoke of a hijacking case against him, but then assured him that he was not the real "target."

"The agents indicated they wanted 'politicians,' " Masselli, who is serving a seven-year federal prison term, said in the affidavit. " 'This is the year of the politician,' an agent said to me . . . . The agents further indicated that . . . if I would help them with incriminating information about politicians, they would assist me in disposing of my hijacking case."

According to Iannuzzi, Masselli told the FBI men that he did not know what they were talking about and drove off. The investigation was dropped in the fall of 1981.

Prosecutors for Bronx District Attorney Mario Merola began their investigation last year and obtained access to the 857 FBI tapes last February. Donovan, Masselli and eight others were later indicted on larceny and fraud charges in connection with a $186 million New York City subway project undertaken by Donovan's firm.