Words from the first black U.S. president and bell ringing around the world on Wednesday will mark 50 years to the minute that civil rights leader Martin Luther King ended his landmark "I have a dream" speech.
Capping a week long celebration of King's historic call for racial and economic justice, President Barack Obama will speak at the Lincoln Memorial, site of King's address on Aug. 28,1963.
The "Let Freedom Ring and Call to Action" ceremony comes as almost half of Americans say much more needs to be done before the color-blind society King envisioned is realized.
Former Presidents Jimmy Carter and Bill Clinton also will address the crowd at the ceremony, which includes bell-ringing at 3 p.m. EDT (1900 GMT), 50 years to the minute after King ended his clarion call of the civil rights movement with the words "let freedom ring."
About 50 U.S. communities or organizations have said they will ring bells. The Swiss city of Lutry and Tokyo are also taking part, said Atlanta's King Center, one of the event's organizers.
Other organizers include the National Action Network of civil rights leader Al Sharpton, the Southern Christian Leadership Conference and the National Council of Churches.
The ceremony follows an interfaith service at Shiloh Baptist Church in Washington, organizers said.
Obama's address will wrap up more than a week of Washington events around the anniversary.
They included a march on Saturday that drew thousands of people urging action on jobs, voting rights and gun violence.
King, a black clergyman and advocate of non-violence, was among six organizers of the 1963 "March on Washington for Jobs and Freedom," where he made his address.
King's speech is credited with helping spur passage of sweeping civil rights laws.
A white prison escapee assassinated the Nobel Peace Prize winner in 1968.