The four-day event, which begins Wednesday, comes less than a month after the Supreme Court struck down a key provision of the Voting Rights Act and not long after the man who shot Martin was acquitted in the racially charged case.
The league is one of the nation's oldest civil rights organizations. The conference, titled "Redeem the Dream: Jobs Rebuild America," is expected to attract about 6,000 people and also focus on strengthening economic opportunities for African-Americans, CEO Marc Morial said.
Holder will talk Thursday about the high court's controversial ruling, which threw out a provision requiring all or parts of 15 states with histories of voting discrimination - mainly in the South- to get federal approval before changing the way they hold elections.
Martin's mother, Sybrina Fulton, will speak Friday. She supports Holder's decision to investigate whether her son's killer, George Zimmerman, could be charged under federal civil rights laws. A jury acquitted Zimmerman in Martin's death, finding that he acted in self-defense.
In both discussions, "what we're hoping they will produce is certainly more knowledge, more information and more mobilization around the issues of voting rights and also justice," said Philadelphia Urban League President Patricia Coulter.
Participants will also commemorate the 50th anniversary of the March on Washington, which protested racial discrimination, and the 150th anniversary of the Emancipation Proclamation, which declared slaves to be free.
A copy of the historic document, signed by President Abraham Lincoln, will be on display at the National Constitution Center in Philadelphia beginning Wednesday. It will remain on view through Sept. 22.
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