The final refrain of Martin Luther King Jr.'s most famous speech will echo around the world as bells from churches, schools and historical monuments "let freedom ring" in celebration of a powerful moment in civil rights history.

Organizers said people at more than 300 sites in nearly every state will ring their bells at 3 p.m. their time Wednesday or at 3 p.m. EDT, the hour when King delivered his "I Have a Dream" speech in Washington.

Commemorations are planned from the site of the speech in Washington to the far reaches of Alaska, where participants plan to ring cow bells along with church bells in Juneau.

On Aug. 28, 1963, as King was wrapping up his speech at the Lincoln Memorial, he quoted from the patriotic song, "My Country 'tis of Thee."

King implored his audience to "let freedom ring" from the hilltops and mountains of every state in the nation, some of which he cited by name in his speech.

"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 in closing.

On Wednesday, bells will answer his call from each of the specific states King named, as well as at other sites around the nation and the world. At the Lincoln Memorial, President Barack Obama and former presidents Bill Clinton and Jimmy Carter will join members of the King family and Georgia Rep. John Lewis, who also spoke at the March on Washington, in ringing a bell that hung in the 16th Street Baptist Church in Birmingham, Ala., before the church was bombed in 1963, organizers said.

International commemorations will be held at London's Trafalgar Square, as well as in the nations of Japan, Switzerland, Nepal and Liberia. London Mayor Boris Johnson has said King's speech resonates around the world and continues to inspire people as one of the great pieces of oratory.

"The response to our call to commemorate the March on Washington and my father's 'I Have a Dream' speech has been overwhelming," King's daughter, the Rev. Bernice King, said in a written statement.

Some of the sites that will host ceremonies are symbolic, such as the Brown v. Board of Education National Historic Site in Topeka, Kan., a monument to the landmark Supreme Court case that outlawed segregated schools in 1954. Bells will also be rung at Lookout Mountain in Tennessee and Stone Mountain in Georgia, a site with a Confederate memorial that King referenced in his speech.

In the nation's capital, numerous organizations and churches will ring their bells at 3 p.m., including the Smithsonian Institution on the National Mall. Washington National Cathedral will play a series of tunes and spirituals on its carillon from the church's central bell tower, including "Lift Every Voice and Sing," ''Mine Eyes Have Seen the Glory," ''Amazing Grace," ''We Shall Overcome" and "My Country 'tis of Thee."

The Very Rev. Gary Hall, the cathedral's dean, said bell ringing is a symbol of freedom in the nation's history and that many churches are trying to answer King's call to be faithful to the roots of the civil rights movement.

"It's a kind of proclamation of our aspirations for the expansion of freedom for all people," he said. "It's always important to remember that the civil rights movement started largely as a church movement. ... It was essentially a group of black clergy with some white allies."

King preached his final Sunday sermon at the National Cathedral in 1968 before traveling on to Memphis, Tenn., where he was assassinated. King had been turning his attention more toward economic inequalities with his Poor People's Campaign, moving beyond solely racial issues to talk about all poor people and high unemployment.

"My feeling is that 50 years later, we need to look at ourselves and our own diversity and our own need to be more open and inclusive and diverse than we have been historically," Hall said. The anniversary is a reminder, he said, "of what a powerful moment that march was in American history and how it really calls us to try to keep faith with the work that was begun 50 years ago."

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