Obama, the first black US president, led civil rights pioneers in a ceremony marking the 50th anniversary of the March on Washington, where King’s speech roused the 250,000 people who rallied there for racial equality.
Obama spoke to thousands of marchers on Washington’s National Mall.
Obama said King’s speech inspired millions of Americans to fight for a more just society and rights that people now take for granted.
“To dismiss the magnitude of this progress, to suggest, as some sometimes do, that little has changed, that dishonours the courage, the sacrifice of those who paid the price to march in those years,” Obama said.
“But we would dishonour those heroes as well to suggest that the work of this nation is somehow complete,” he said, calling economic justice the “unfinished business” of the civil rights battle.
Marchers, many wearing T-shirts with King’s face on them, began their walk near the US 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.
Fighting restrictive voting rights laws that Democrats say hurt minorities, combating joblessness, and reducing gun violence among African-Americans are among the issues that civil rights leaders put at the forefront of their efforts in 2013.
“This march was supposed to be about jobs, but it’s about a lot more,” said marcher Ash Mobley, 27, of Washington, who said she was there to represent her grandmother, who had been at the 1963 event.
The marchers heard speeches from former presidents Bill Clinton and Jimmy Carter as well as members of King’s family on the steps of the Lincoln Memorial, the site of King’s address on Aug 28, 1963.
A bell rang at 3pm (7pm Irish time), 50 years to the minute after King ended his clarion call of the civil rights movement with the words “Let freedom ring”.
Bernice King, his youngest child, urged the crowd to stay true to the ideals enunciated by her father.
“If freedom is going to ring in Libya, in Syria, in Egypt, in Florida, then we must reach across the table, feed each other, and let freedom ring,” she said.
Obama has said the country’s history of racial discrimination had contributed to a persistent economic gap between black and white people in the decades since King’s speech.
Obama, whose mother was white and whose father was black, has sometimes seemed reluctant to weigh in on persistent racial divides in the US, but he spoke forcefully about the issue last month after the man who killed black Florida teenager Trayvon Martin was acquitted.
The “Let Freedom Ring and Call to Action” ceremony comes as almost half of Americans say much more needs to be done before the colour-blind society King envisioned is realised.
Yesterday’s event caps a week-long celebration of King’s historic call for racial and economic justice. They included a march on Saturday that drew thousands of people urging action on jobs, voting rights, and gun violence.
Oprah Winfrey said Martin Luther King Jr forced the US “to wake up, look at itself and eventually change”.
The TV personality, right, said the civil rights leader’s lessons continue to inspire people all over the world.
Winfrey said Dr King recognised that Americans shared the same dreams and that their hopes were not different based on race.
She said King was right when he said all Americans’ destinies are intertwined and would rise or fall based on how people treat their neighbours.
Winfrey said she asked her mother as a nine-year-old girl why her family was not there for the march. Winfrey said it took 50 years, but she finally arrived on the steps of the Lincoln Memorial to mark the anniversary of King’s march.