LOS ANGELES -- Promising to lead “without regard to politics or partisanship," Jackie Lacey was sworn in Monday as Los Angeles County’s first female and first African American district attorney.

Lacey, 55, giggled briefly as she raised her right hand for the ceremony in front of an estimated 1,200 people -- including elected officials, judges and prosecutors, as well as relatives and well-wishers -- at USC's Galen Center.

"I'm nervous," she said before her predecessor, Steve Cooley, administered the oath to become the county's top prosecutor.

Cooley, who retired after a historic three terms and backed Lacey to succeed him, described the handover as the end of one era and the start of a new one.

In her inaugural address, Lacey said the 1,000 prosecutors she will lead in her new role were "my heroes," and she drew on the experience of her parents, who escaped the racism of the South to build a new life in Southern California.

"I will start my day as I have done for many years, asking God to use me as his servant and make me the type of leader that others will want to follow,” she told the crowd, who gave her a standing ovation.

Superior Court Judge Lee Smalley Edmon, the county's first female presiding judge, hailed Lacey's election as evidence of the progress women have made in the legal profession in recent years.

She noted that women hold the positions of California attorney general and chief justice of the state Supreme Court and that a majority of county prosecutors are female.

"Jackie is a huge role model for countless young girls and people of color," Edmon said. "Now the D.A.'s office can no longer be considered a male-only position."

Lacey, then Cooley's chief deputy, beat veteran prosecutor Alan Jackson in last month's run-off election.

As district attorney, Lacey now heads the largest local prosecutorial office in the nation and the most powerful in the county's criminal justice system -- one responsible for prosecuting roughly 60,000 felony cases a year, including murders, rapes and robberies.

Lacey takes over the office at a time when the state's criminal justice system is facing difficult challenges, including budget cuts, overcrowded prisons and realignment -- the shifting of responsibility for some mostly nonviolent criminals from state prisons to county jails.