The first black US President will today speak to commemorate the moment civil rights leader Martin Luther King ended his landmark “I have a dream” speech 50 years ago.
Bringing to a close a seven day celebration of King's historic call for racial and economic justice, President Barack Obama will speak on the steps of the Lincoln Memorial, the site of King's address watched by 250,000 people on 28 August, 1963.
Former Presidents Jimmy Carter and Bill Clinton will in turn address the audience at the ceremony, which includes bell-ringing at 3 pm 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."
The "Let Freedom Ring and Call to Action" ceremony will involve a march through downtown Washington, which is expected to draw up to 100,000 participants, followed by an interfaith service at Shiloh Baptist Church in Washington, organisers said.
King, a black clergyman, non-violent activist and leader in the African-American civil rights movement was one of six organisers of the 1963 "March on Washington for Jobs and Freedom," where he gave his speech. He received the Nobel Peace Prize in 1964 for combating racial inequality through non violent means.
King's speech is credited with helping spur passage of sweeping civil rights laws.
King was assassinated in1968 after being shot in the Lorraine Motel in Memphis on 4 April.
Approximately 50 US communities and organisations have said they will ring bells. The Swiss city of Lutry and Tokyo are also taking part, said Atlanta's King Center, another of the event organisers said.
The National Action Network of civil rights leader Al Sharpton, the Southern Christian Leadership Conference and the National Council of Churches are also involved in preparations.
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.
Additional reporting by agencies