Testifying under heavy security prompted by a series of death threats, the chief accuser of former labor secretary Raymond J. Donovan invoked the Fifth Amendment 146 times today concerning his role as an FBI informer on the Mafia.

Michael Orlando, 44, said he was acting on the advice of his lawyer, who charged that the Mafia was trying to kill Orlando and that FBI officials were trying to "bury" him legally.

The lawyer, David S. Gould, made his charges in an impromptu news conference punctuated at times by angry outbursts from Donovan, who was standing nearby.

Orlando was the catalyst for the FBI surveillance underlying the case against Donovan, and as a secret informer he made a number of allegations that have dogged Donovan since his nomination to the Reagan Cabinet in 1980.

Donovan and his company, Schiavone Construction, are accused of defrauding the New York City Transit Authority of $7.4 million on a subway project. Donovan has denied the allegations.

Orlando's testimony, however, is not likely to be needed if the Donovan case goes to trial in New York Supreme Court in the Bronx. Today's pretrial hearing was one in a series over the legitimacy of the FBI tapes that are the main evidence in the case.

Gould emphasized that he had "not one iota of evidence" that Donovan or his company was "behind these obstructions" that kept Orlando from testifying today.

Donovan, however, took sharp exception to Gould's portrayal of his client as a credible witness who had done his best to cooperate with authorities in the past. At one point, the former labor secretary thrust a newspaper into Gould's hands, showing a headline that said "FBI to Curb 'Hearsay' Disclosures."

"That comes four years too late for me and my family," Donovan said in bitter tones. He called Orlando "a pathological liar."

The FBI tapes in the Donovan case were compiled in 1979 in court-ordered eavesdropping at a South Bronx warehouse, partly because of information supplied by Orlando, partly on the basis of a Nov. 30, 1978, truck hijacking there that FBI agents observed.

Defense lawyers contend that Orlando engineered the hijacking to provide the FBI with probable cause for the electronic surveillance and that Orlando's FBI handlers concealed such information from the court.

The two FBI agents who dealt with Orlando, who has grown a luxuriant beard since entering the witness protection program in 1983, have sworn they had no such knowledge. Orlando invoked his Fifth Amendment privilege against self-incrimination when asked whether he committed the hijacking and what he told the FBI about it.

Orlando first became Donovan's accuser in an anonymous way in January 1981, on the eve of Donovan's confirmation hearings.

Although "discontinued" as an informer by then, Orlando told his main FBI contact that he recognized Donovan as a man he had chauffeured to a meeting with organized crime figures at a Miami Beach delicatessen during the 1979 Super Bowl weekend there.

Orlando said he picked Donovan up as a driver for William P. Masselli, the owner of the Bronx warehouse and a reputed Mafia member who had gone into business as a subcontractor for Donovan's firm.

Gould charged today that "some" federal officials are bent on prosecuting Orlando, claiming that one FBI official, Kenneth Walton, "wants to bury him [Orlando]."

Walton said through an FBI spokesman that "he never said anything like that."