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President Obama said Wednesday that the iconic words of the Rev. Martin Luther King Jr. “belonged to the ages,” as he urged Americans to “keep on marching” to fulfill his dream. The nation’s first black president spoke 50 years to the minute after King delivered his iconic “I Have a Dream” speech, in the same spot at the steps of the Lincoln Memorial.

Obama offered his personal reflections on the civil rights movement, King’s speech, the progress achieved and the challenges that continue to face the nation.

Obama spoke often of the “courage” it took for civil rights leaders like King, Medgar Evers, James Chaney, Andrew Goodman and Michael Schwerner to make a difference.

“They did not die in vain,” Obama told the crowd. “Their victory was great, but we would dishonor those heroes as well to suggest that the work of this nation is somehow complete.”

Obama encouraged Americans of all colors and faiths to “keep on marching.”

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    “Because they kept marching, America changed. Because they marched the civil rights law was passed. Because they marched, a voting rights law was signed. Because they marched, doors of opportunity and education swung open so their daughters and sons could finally imagine a life for themselves beyond washing somebody else's laundry or shining somebody else's shoes. Because they marched, city councils changed and state legislatures changed and Congress changed and yes, eventually the White House changed,” Obama said.

    Obama, as well as former presidents Bill Clinton and Jimmy Carter, were among the headliners in D.C. Wednesday to commemorate the 50th anniversary of the March on Washington.

    "This march, and that speech, changed America," Clinton declared, remembering the impact on the world and himself as a young man. "They opened minds, they melted hearts and they moved millions -- including a 17-year-old boy watching alone in his home in Arkansas."

    Carter said King's efforts had helped not just black Americans, but "in truth, he helped to free all people."

    Former President George W. Bush, who was invited but is still recovering from a heart procedure and will not attend, said in a written statement that “our country has come a long way since that bright afternoon 50 years ago; yet our journey to justice is not complete.”

    Oprah Winfrey, who also spoke at the ceremony, said King forced the nation "to wake up, look at itself and eventually change." She also said the civil rights leader's lessons inspire people all over the world.

    Winfrey also said King recognized that Americans shared the same dreams and that their hopes weren't different based on race. She says King was right when he said all Americans' destinies are intertwined and would rise or fall based on how people treat their neighbors.

    Performers also included Peter Yarrow and Noel Paul Stookey, their voices thinner now than when they performed at the original march as part of the folk trio Peter, Paul and Mary. They sang "Blowin' in the Wind," as the parents of slain black teenager Trayvon Martin joined them on stage and sang along.

    In a radio interview Tuesday with Tom Joyner and co-host Sybil Wilkes of the Tom Joyner Morning Show, Obama said he imagines that 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.

    "When you are talking about Dr. King's speech at the March on Washington, you're talking about one of the maybe five greatest speeches in American history," Obama said.  "And the words that he spoke at that particular moment, with so much at stake, and the way in which he captured the hopes and dreams of an entire generation I think is unmatched."

    In tribute, Obama keeps a bust of King in the Oval Office and a framed copy of the program from that historic day when 250,000 people gathered for the March on Washington for Jobs and Freedom.

    On Saturday, tens of thousands of people marched to the Martin Luther King Jr. Memorial and down the National Mall to commemorate the anniversary of the speech.

    The event was in homage to a generation of activists who endured fire hoses, police abuse and indignities to demand equality for African Americans, and to the thousands who marched in the peaceful demonstration on Aug. 28, 1963.

    An estimated 200,000-300,000 people participated in the March on Washington to demand greater civil rights for African Americans.  In his famous speech, King called for the end of racial injustice in the U.S.
    The march and its effects are credited with helping pass the Civil Rights Act in 1964 and the Voting Rights Act in 1965, both of which insured equal rights for all U.S. citizens.

    The Associated Press contributed to this report.