Mary Lou Leary, a veteran federal prosecutor, was sworn in yesterday as interim U.S. attorney for the District, replacing Eric H. Holder Jr. and marking the first time a woman has held the job in Washington.

Holder, who had been the District's top prosecutor since 1993, was unanimously confirmed by the Senate on Thursday night as deputy attorney general and sworn in yesterday as second in command at the Justice Department. Leary will take his place until a permanent replacement is selected and confirmed.

"I'm thrilled," she said yesterday. "I love this office. I've been here for 12 years, and I have the utmost respect and admiration for what Eric Holder has done. I want to continue the tradition he has established."

Leary, 47, headed Superior Court operations for the U.S. attorney's office. She now will oversee a staff of 300 lawyers and an equal number of support personnel. The District has the nation's largest federal prosecutor's office. Prosecutors deal with everything from minor street crimes to terrorist incidents, international conspiracies and intelligence-related investigations.

Leary has not applied for the job on a permanent basis. Her appointment was made by Attorney General Janet Reno and hailed by Holder, who said she will provide "energetic, sound and thoughtful leadership" during the transition. Dozens of colleagues attended her swearing-in, administered by Chief U.S. District Judge John Garrett Penn, who noted the new milestone for women.

Del. Eleanor Holmes Norton (D-D.C.) has yet to recommend a replacement for Holder, even though his move to the Justice Department has been in the works since March. She is awaiting advice from a nonpartisan citizen commission that is screening candidates. Norton said yesterday that the commission has needed extra time to review the records of applicants and that she expects the panel to submit names to her within the next few days.

Other sources said three front-runners have emerged: J. Ramsey Johnson, now principal assistant U.S. attorney, who was interim U.S. attorney for six months before Holder was appointed to the job in 1993; Sharon A. Sprague, now executive assistant U.S. attorney; and Wilma A. Lewis, a former supervisor at the U.S. attorney's office who now is inspector general of the Interior Department.

Norton has declined to say when her choice will be revealed. Her recommendation then goes to President Clinton, who sends his choice to the Senate for confirmation.

This is the second time that Norton has been involved in the selection procedures. The same process resulted in Holder's selection. Before that, the White House traditionally made choices with little community input.

Leary has tried scores of misdemeanor and felony cases and held numerous supervisory and management positions. She has headed Superior Court operations since July 1995 and often meets with community leaders. CAPTION: Mary Lou Leary, interim U.S. attorney for the District, stands in front of courthouse.