President Barack Obama's presence at the ceremony celebrating the 50th anniversary of the March on Washington will embody the dreams of the 250,000 people who rallied there decades ago for racial equality - and the continuing struggle by others for that goal.
The first black U.S. president is expected to speak just after 20.00BST , the time when the Rev. Martin Luther King Jr. delivered his spellbinding "I Have a Dream" speech early in the turbulent 1960s. The landmark Civil Rights Act and Voting Rights Act to outlaw discrimination were signed into law in the next two years.
Mr Obama has said Dr King is one of two people he admires "more than anybody in American history." The other is Abraham Lincoln. The president will speak at the Lincoln Memorial. He will be joined by former presidents Jimmy Carter and Bill Clinton.
Mr Obama will offer his personal reflections on the civil rights movement, Dr King's speech, the progress achieved in the past 50 years and the challenges that demand attention from the next generation.
International commemorations will be held in London's Trafalgar Square, as well as in Japan, Switzerland, Nepal and Liberia. London Mayor Boris Johnson has said Dr King's speech resonates around the world and continues to inspire people as one of the great pieces of oratory.
On August 28, 1963, as Dr King was ending his speech, he quoted from the patriotic song, "My Country 'tis of Thee" and urged his audience to "let freedom ring."
"When we allow freedom to ring - when we let it ring from every city and every hamlet, from every state and every city, we will be able to speed up that day when all of God's children, black men and white men, Jews and Gentiles, Protestants and Catholics, will be able to join hands and sing in the words of the old Negro spiritual, 'Free at last, free at last, great God almighty, we are free at last," he said. The civil rights leader was assassinated five years later.
Mr Obama considers the 1963 march part of his generation's "formative memory." A half-century after the march, he said, is a good time to reflect on how far the country has come and how far it still has to go, particularly after the recent acquittal of George Zimmerman in the fatal shooting of Trayvon Martin, an unarmed black teenager.
In a radio interview he said he imagines that Dr King "would be amazed in many ways about the progress that we've made." He listed advances such as equal rights before the law, an accessible judicial system, thousands of African-American elected officials, African-American CEOs and the doors that the civil rights movement opened for Latinos, women and gays. "I think he would say it was a glorious thing," he said.
But Mr Obama noted that Dr King's speech was also about jobs and justice. "When it comes to the economy, when it comes to inequality, when it comes to wealth, when it comes to the challenges that inner cities experience, he would say that we have not made as much progress as the civil and social progress that we've made, and that it's not enough just to have a black president," he said.