Civil rights leader King gave his famous speech at an event that drew over 250,000 people to Washington's Lincoln Memorial on Aug. 28, 1963. He envisioned a nation transformed into an oasis of justice, free from racial inequality.
"Now is the time to rise from the dark and desolate valley of segregation to the sunlit path of racial justice," said King — words that influenced the then-Kennedy administration to accelerate racial equality legislation.
Obama commented on the bravery of the people who came together for the march.
"They came by the thousands, from every corner of our country," Obama said Wednesday. "Men and women, young and old, blacks who longed for freedom and whites who could no longer accept freedom for themselves while witnessing the subjugation of others."
Obama delivered his address on the steps of the same memorial after the ringing of bells at 3 p.m. ET, to mark the time that King began to speak to people lining the National Mall.
"Because they kept marching, America changed," Obama said.
"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 their sons could finally imagine a life for themselves beyond washing somebody else's laundry or shining somebody else's shoes."
Former U.S. presidents Jimmy Carter and Bill Clinton took the stage earlier in the day.
Clinton praised King's famous speech, but he also lauded the people who gathered for the march in 1963, saying "what a debt we owe to those people who came here 50 years ago."
"This march and that speech changed America," Clinton said. "They opened minds, they melted hearts, and they moved millions, including a 17-year-old boy watching alone in his home in Arkansas.
"It was an empowering moment, but also an empowered moment."
The mayor of Hattiesburg, Miss., Johnny DuPree, told the crowd in Washington on Wednesday that because of King, he was able to become mayor of the fourth largest city in Mississippi after humble beginnings, when as a boy he played baseball in the streets with rocks "because my mom couldn't afford a ball."
"We're here today because of people like King, who did not quiver or retreat in the face of injustice," he said.
Not everyone at the latest march was celebrating progress. "I thought we would be a lot further along than we are 50 years after hearing King's speech," said John Pruitt, 83, a voter rights advocate who attended the first march as well.
'How will the dream live on'
Oprah Winfrey, one of the speakers at the commemorative event, asked people to think about King's legacy and how they can serve society.
"As the bells toll today, let us reflect on the bravery, let us reflect on the sacrifice of those who stood up for freedom, who stood up for us, whose shoulders we now stand on," Winfrey said.
"As the bells toll today at three, let us ask ourselves: How will the dream live on in me? In you? In all of us? As the bells toll, let us remind ourselves, injustice anywhere is a threat to justice everywhere."
A number of speakers said while King made great strides in moving civil rights forward, there is much work to be done.
Civil rights activist Myrile Evers-Williams told the crowd the movement for social equality "can no longer afford an individual approach," but is an "interconnected struggle" for all demographics.
Several of King's family members spoke, recalling the speech and the meaning it had for them and others. King's family members will ring a bell at the Lincoln Memorial to mark an unforgettable moment in his career.
Read tweets and messages in our CBC live blog below as people share memories and describe how the 1963 speech touched their lives.