U.S. President Barack Obama remembered the power of Martin Luther King Jr.'s iconic "I have a dream" speech  at a ceremony in Washington today, saying the "hunger of purpose" that motivated thousands of people to join the March on Washington 50 years ago lives on in this generation.

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, the first black U.S. president, commented on the bravery of the people who came together for the march.

"They came by the thousands, from every corner of our country," he said. "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."

At precisely 3 p.m., members of the King family tolled a bell to echo King's call 50 years earlier to "let freedom ring." It was the same bell that once hung in the 16th Street Baptist Church in Birmingham, Ala., where four black girls were killed when a bomb planted by a white supremacist exploded in 1963.

When King made his famous speech, the South was still segregated, with separate restrooms and schools for blacks and whites.

It took two years until Lyndon Johnson — who became president following the assassination of John F. Kennedy just three months after King's speech — finally prevailed over Congress to sign the country's landmark Civil Rights Act and Voting Rights Act into law. King was assassinated three years after that, in 1968.

King's dream 'elusive'

Obama remarked on the powerful changes that have moved through America over the past five decades, but he pointed to the nation's economic disparities as evidence that more needs to be done to realize King's hopes.

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Former Presidents Jimmy Carter, Bill Clinton, first lady Michelle Obama and President Obama listen to the national anthem during the commemoration of the 50th anniversary of the March on Washington and Martin Luther King Jr.'s 'I have a dream' speech. (Kevin Lamarque/Reuters)

Obama said there have been "examples of success within black America" that would not have been possible 50 years ago, but noted that black unemployment rates are still almost twice as high as white unemployment rates and the gap in wealth between races has grown. The position of "all working Americans" has eroded over the years, Obama said, "making the dream Dr. King described even more elusive."

Obama said people today may not face the same dangers as in 1963, but the "fierce urgency of now remains."

"We may never duplicate the swelling crowds and dazzling procession of that day so long ago – no one can match King's brilliance – but the same flame that lit the heart of all who are willing to take a first step of justice, I know that flame remains."

Former U.S. presidents Jimmy Carter and Bill Clinton took the stage earlier in the day, two speakers in a lengthy ceremony that included civil rights activists, members of King's family and celebrities like Oprah Winfrey.

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.

Earlier in the day, marchers, many wearing T-shirts with King's face on them, began their walk near the U.S. Capitol.

They were led by a line of military veterans and people who had been at the 1963 march, their arms linked. People sang We Shall Overcome and other civil rights anthems.

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.

D'Army Bailey, an activist who grew up in Memphis, Tenn., and attended a segregated high school, attended the original march when he was 22. He was tossed from his university for his role in the civil rights movement, but he went on to become a lawyer, judge and founder of a national civil rights museum.

Bailey told CBC's As It Happens that he had mixed emotions about the anniversary celebrations, noting that economic disparities and battles over voting continue.

"While I think today is a great day to celebrate, I think that we must be sober in our assessment, as I think we should have been even at the time of the march 50 years ago," he said.

He said the commemoration provided a time to pause and acknowledge the "courage of the foot soldiers of the movement" who put their lives on the line for civil rights.

Read tweets and messages in our CBC live blog below as people share memories and describe how the 1963 speech touched their lives.

With files from The Associated Press and Reuters