The president spoke at the Lincoln Memorial on Wednesday on the 50th anniversary of the 1963 March on Washington. Tens of thousands of people flooded the National Mall to commemorate King's "I Have a Dream" speech.
Mr. Obama claimed his place in King's dream, as he held himself up as a symbol of the change King envisioned. But he also pointed to the nation's lingering economic disparities as evidence that King's hopes remain unfulfilled.
With Biblical references and the cadences of a preacher, Mr. Obama used the refrain, "because they marched," as he recited the achievements of the civil rights movement.
Two former presidents, Bill Clinton and Jimmy Carter, spoke movingly of King's legacy - and of problems still to overcome.
"This march, and that speech, changed America," Clinton said, recalling 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." Still, Carter listed a string of current events that he said would have spurred King to action today, including the proliferation of guns and stand-your-ground laws, a Supreme Court ruling striking down parts of the Voting Rights Act, and high rates of joblessness among blacks.
The anniversary not only marked King's famed speech, it commemorated the historic March on Washington for Jobs and Freedom, one of the largest political rallies for human rights in American history and one of the most monumental events in the civil rights struggle.
On Aug. 28, 1963, tens of thousands of blacks and whites poured into the nation's capitol by the busloads, converging around the monument of the president who freed the slaves. The demonstrators' signs and voices demanded racial equality.
Those who attended the original march say the love and support was palpable.
"Looking around me and seeing people from all walks of life, old and young. There were more white people there than many of us expected, and it was wonderful to see Americans turning out to work for justice, jobs and freedom," said civil rights activist Peggy Dammond Preacely.
Wednesday's ceremony was meant to be a celebration of progress and a call to action on new challenges.
At 3 p.m. ET, church bells rang out at the National Cathedral and nationwide to mark the moment of King's speech exactly 50 years ago.
On the National Mall, Mr. Obama joined members of King's family to ring a bell that hung in the 16th Street Baptist Church in Birmingham, Ala., before the church was bombed in 1963 just weeks after King's famous speech.
Bell ringing commemorations were planned across the country and around the world. They were meant to answer King's closing refrain to "let freedom ring" for all people.
Stars, including singer Usher, contributed to "The Dream at 50," a video played simultaneously at L.A. Live in downtown Los Angeles and cities around the world.
The Associated Press contributed to this report.