Seth William was sworn in today as Philadelphia's first African-American district attorney and pledged to improve conviction rates in the city.

"Four years from now, we won't have the lowest conviction rate in the country," he said.

Saying he wanted "to make Philadelphia the safest big city in America," Williams to the oath of office to become the first new chief prosecutor in the city in 18 years.

He replaces Lynne M. Abraham, who decided not to seek re-election in November.

Williams, 42, had worked for Abraham as an assistant district attorney and sought unsuccessfully to unseat her four years ago.

He was sworn in a ceremony at the Kimmel Center's Verizon Hall this morning, along with the re-elected City Controller Alan L. Butkovitz and 28 judges.

A federal study of conviction rates in 39 large urban counties has ranked Philadelphia last in two consecutive reports.

During the swearing-in, Williams and others mentioned an Inquirer series that went beyond that study in an analysis of 31,000 criminal cases from 2006 to 2008, finding that Philadelphia defendants walk free on all charges in nearly two-thirds of violent-crime cases.

"The Philadelphia criminal justice system is broken," Williams said this morning, despite the hard work of "dedicated and underpaid public servants who toil hard every day . . . "

Williams said he would ask Mayor Nutter and City Council for more funds to help him prevent and prosecute crime.

Abraham, who also attended the ceremony, said she would announce plans tomorrow to join a law firm but would not identify it today. She also said she likely would run for office in the future, but would not say which one.