US President Barack Obama is set to lead civil rights pioneers in a ceremony for the 50th anniversary of the March on Washington. Dr Martin Luther King's "I Have a Dream" speech roused the 250,000 people who rallied there decades ago for racial equality. Large crowds gathered at the Lincoln Memorial, where the first black US president is expected to speak just after 1900 GMT (0500 AEST) - the same time that King delivered his spellbinding speech. The first march was early in the turbulent 1960s, when the South still had separate restrooms, schools and careers for blacks and whites, and racism lingered across the country. In the two years following the march, President Lyndon Johnson signed the landmark Civil Rights Act and Voting Rights Act to outlaw discrimination, and King received the Nobel Peace Prize. Obama will be joined by former Presidents Jimmy Carter and Bill Clinton, with thousands of people expected to attend. International commemorations will be held at London's Trafalgar Square, as well as in the nations of Japan, Switzerland, Nepal and Liberia. On August 28, 1963, as 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," King said. The civil rights leader was assassinated five years later.