Attorney General Benjamin R. Civiletti announced yesterday that he was assigning a top federal prosecutor to head the investigation of leaks from the FBI's undercover operation that has allegedly implicated eight members of Congress in potential bribe cases.

Civiletti told a National Press Club audience he had asked Richard Blumenthal, 33, the U.S. attorney in Connecticut, to direct the inquiry because he was from outside Justice Department headquarters and had no association with the operation, code-named Abscam, for Arab scam.

The attorney general said that nothing in his tenure at the department "has angered or frustrated me more than the revelations" about the sting operation in which FBI undercover agents posed as representatives of Arab businessmen and discussed cash payoffs to public officials for legislative favors.

"There is no excuse for violating the rights of people who are entitled to be presumed innocent, especially public officials whose reputations are their life blood" he said. He added that no other members of Congress are under investigation in the Abscam case.

Civiletti also defended the tactics used in the operation, saying, "Crimes were not created by the FBI at all. They wre following leads . . . and had a basis to believe criminal acts were occuring or had occurred."

He did acknowledge that at times in undercover operations "misrepresentations" will be made by the middlemen and con artists who bring people into the net.

For example, sources familiar with the investigation said it was clear when the middleman brought Sen. Larry Pressler (R-S.D.) to videotape cameras in a Washington house that the senator was not aware of any deals. So the agents made no overtures to Pressler.

Civiletti said he couldn't guarantee that "entrapment" wasn't used in the operation, but noted that the legal questions in the case had been closely and carefully reviewed by FBI Director William H. Webster and Philip B. Heymann, head of the criminal division at the Justice Department.

He said only future battles in court will show whether anyone involved in the undercover work crossed the line of legality and enticed one of the suspects.

Blumenthal declined to answer reporters' questions after the luncheon. A department spokesman said later that he will report directly to Civiletti while consulting with Michael E. Shaheen Jr., head of the department's internal investigation unit, the office of professional responsibility.

The Blumenthal appointment is being viewed in the department as yet another sign that Civiletti takes this leak investigation seriously. Several Justice and FBI officials with access to Abscam documents have already given sworn affidavits in the leak investigation, sources said.

It has also been reported that Justice investigators have not ruled out the use of lie detector tests.

Blumenthal became U.S. attorney in Connecticut about a year after he left the staff of the state's senior senator, Abraham A. Ribicoff. He also worked for the Senate campaign of Daniel P. Moynihan (D-N.Y.) in 1976 and for a short time in 1970 on Moynihan's staff in the Nixon White House.

He is a graduate of Harvard, and Yale Law School, and was for a brief period in the late 1960s a reporter for The Washington Post and an assistant to then publisher Katharine Graham.

On other topics yesterday, Civiletti said:

The Justice Department is working on guidelines for ensuring as far as possible that trials and pretrial proceedings are held in public, as a counter to a controversial Supreme Court ruling last year.

He opposed a blanket exemption from the Freedom of Information Act for the Central Intelligence Agency.