"They did not die in vain; their victory was great," Barack Obama said of King and other slain civil rights leaders of the tumultuous 1960s as he delivered his address just after 3 p.m. on Wednesday, precisely the same time that King did on Aug. 28th a half-century ago.
"But we would dishonour those heroes as well to suggest that the work of this nation is somehow complete. The arc of the moral universe may bend towards justice, but it doesn't bend on its own. To secure the gains this country has made requires constant vigilance, not complacency."
Obama's remarks capped a rainy day of events in the U.S. capital that began when as many as 100,000 people marched through the summer drizzle to the National Mall, an expanse of public parkland that links the Capitol building in the east to the Lincoln Memorial in the west.
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.
On Wednesday, Obama and his wife, Michelle, walked down the steps of the Lincoln Memorial past a cast-iron bell from a Baptist church in Birmingham, Ala., where four black girls were killed by a bombing just three weeks after King made his speech.
Obama and many of the event's participants — VIPs that included Oprah Winfrey, members of King's family, Caroline Kennedy and former presidents Jimmy Carter and Bill Clinton — noted that while King called for equality and justice for all in 1963, African-Americans and other minorities still face significant obstacles.
"Whether it's by challenging those who erect new barriers to the vote or ensuring that the scales of justice work equally for all in the criminal justice system and not simply as a pipeline from underfunded schools to overcrowded jails, it requires vigilance," Obama said to applause from the crowd.
Civil right icon John Lewis, now a congressman from Georgia who was the youngest speaker at King's March on Washington for Jobs and Equality, pointed out that illegal immigrants are currently "hiding in fear," that African-American men are incarcerated at higher rates per capita than any other demographic group, and that unemployment and homelessness continue to plague America.
"Fifty years later, we can ride anywhere we want to ride, we can stay where we want to stay, those signs that said white and coloured are gone, and you won't see them anymore except in a museum, in a book, or on a video," the 73-year-old Lewis said.
"But there are still invisible signs, barriers in the hearts of humankind that form a gulf between us ... We still have a great distance to go before we fulfil the dream of Martin Luther King Jr."
Indeed, in the five decades since King's famous speech, race relations remain a touchy, painful, ongoing concern in the United States.
This summer's acquittal of George Zimmerman, who shot and killed unarmed black teen Trayvon Martin in Florida in 2012, has exacerbated many of those ever-simmering tensions. Martin's name was mentioned several times by speakers at the Lincoln Memorial on Wednesday, including by Lewis.
Obama himself has often served as a flashpoint for race-tinged ire from his foes, particularly in the South.
A recent Public Policy Polling survey suggested a third of those in Louisiana say they even blame Obama for hurricane Katrina, a monster storm that devastated the city of New Orleans three years before he won the Oval Office.
The conservative Daily Caller news website recently maligned Obama for his new dog.
"With the addition of Sunny, the Obamas now have two black Portuguese water dogs," the Daily Caller wrote. "The Obamas do not have any white dogs."
Obama was careful in his remarks on Wednesday to stress the importance of the civil rights movement for all minorities, not just African-Americans. He also once again chided some in the black community, saying justified outrage over racial discrimination has sometimes devolved into "excuse-making for criminal behaviour" and poverty is presented as "an excuse for not raising your child.”
He pointed as well to the country's economic disparities as evidence that some of King's fondest hopes have yet to be realized, urging action and taking a veiled shot at his Republican opponents on Capitol Hill.
"We can continue down our current path in which the gears of this great democracy grind to a halt and our children accept a life of lower expectations, where politics is a zero-sum game, where a few do very well while struggling families of every race fight over a shrinking economic pie," Obama said. "That’s one path.”
Or, he continued, "we can have the courage to change."
There were no Republican speakers at Wednesday's event. Former president George W. Bush was invited, but declined to attend because he's recovering from heart surgery in Texas.
Instead, Bush issued a statement calling on "every American to help hasten the day when Dr. King's vision is made real in every community — when what truly matters is not the colour of a person's skin, but the content of their character."
Bill Clinton, meantime, took direct aim at Republicans, making reference to the party's efforts to restrict voting rights in several states while opposing tougher gun control laws.
"A great democracy does not make it harder to vote than to buy an assault weapon," said Clinton, whose wife, Hillary, is eyeing a run for president in 2016.
He also made a remark about congressional gridlock that caused some pundits to wonder if he was scolding Obama.
"I would respectfully suggest that Martin Luther King did not live and die to hear his heirs whine about political gridlock," he said. "It is time to stop complaining and put our shoulders against the stubborn gates holding the American people back."