"When we look at the state of race relations, we've made dramatic progress in 50 years – but we're nowhere near where we need to be," King's activist son, Martin Luther King III, says.
The United States of America today marked 50 years since the assassination of Martin Luther King Jr, with activists paying tribute to the civil rights leader and reflecting on how 21st-century advocates might carry his legacy forward.
In a country still torn over issues of race and class, thousands of demonstrators rallied in Memphis, Tennessee where the pastor and Nobel Peace Prize winner was slain aged 39 on a motel balcony by a white supremacist sniper on April 4, 1968, as well as in Washington where he delivered his historic "I Have a Dream" speech.
Bells were set to toll 39 times at about 6.01pm local time, the time King was shot, in Memphis and around the nation to honour the icon whose moral courage helped bring lasting changes to US life.
"When we look at the state of race relations, we've made dramatic progress in 50 years – but we're nowhere near where we need to be," King's activist son, Martin Luther King III, told ABC from Memphis, where he took part in a symbolic march.
"He would know that we as a nation can, must and will do better."
Lionised today for his heroic campaigns against racism and segregation, King was a controversial, radical activist who with a mantra of non-violence ardently campaigned against poverty and economic injustice, including what he called the continued "exploitation of the poor," and US wars abroad.
His birthday is a national holiday, and a nine-metre-high statue in his likeness towers in Washington as a tribute to his life and work.
On the anniversary's eve prominent civil rights activist Reverend Jesse Jackson – speaking from Memphis's Lorraine Motel balcony, where King was gunned down – said "the sore is still raw" from the fatal shooting.
"It's always a source of pain and anxiety," said Jackson, who was a member of King's entourage and was at the motel when he was murdered.
"It happened so suddenly, in the middle of a conversation, on the way to dinner. He'll always be 39."
But his legacy, Jackson said, survives in the hearts and actions of demonstrators today wielding flags of racial, social and economic justice.
King catapulted into the national spotlight by taking the lead on a year-long boycott against racial segregation on local buses.
He is perhaps best known for the "I Have a Dream" speech he delivered to some 250,000 demonstrators on August 28, 1963 as part of the "March on Washington for Jobs and Freedom."
One year later he became the youngest Nobel Peace Prize winner at 35 for his non-violent resistance.
Prior to King's assassination, which triggered an outpouring of grief and riots in more than 100 cities, he had traveled to Memphis to support sanitation workers striking for better conditions and higher pay.
Elmore Nickleberry, now 86, is today one of the last participants in that strike still on the job.
"The mood was mighty bad when he got killed. People started hollering, started crying," Nickleberry told AFP.
He recalled that poignant moment of tension and pain, but Nickleberry said it is King's call for non-violent action that lives on.
"That's what I remember today."
King's focus on economic justice was a rallying point Wednesday, as union workers marched for fair wages and activists lamented the concentration of poverty within black communities.
King fought not just against the "Jim Crow laws" that discriminated against blacks, said Nancy Taylor, a lawyer attending the march.
"He also fought against economic injustice, and that was the message that's really been lost in his legacy."
US President Donald Trump paid homage to the civil rights icon by proclaiming April 4, 2018 a day to honour King.
"It is not government that will achieve Dr. King's ideals, but rather the people of this great country who will see to it that our Nation represents all that is good and true, and embodies unity, peace, and justice," Trump said in a statement.
Trump has been sharply criticised for divisive comments targeting Latino and Muslim immigrants, and for refusing to condemn outright a violent white supremacist rally last year that ended in bloodshed.
Several US lawmakers travelled to Memphis for the day-long tribute. Among them was Senator Bernie Sanders, a former presidential candidate, who said "the legacy for us is to follow in (King's) footsteps and to transform this country."
A large crowd gathered at the Lorraine Motel, which has been transformed into the National Civil Rights Museum, while thousands marched near a union headquarters where King had joined the sanitation workers on the eve of his assassination.
"I've seen the promised land. I may not get there with you," King prophetically said that evening. "But I want you to know tonight, that we as a people will get to the promised land!"
Barack Obama, the nation's first black president, paid tribute in a video message, stressing that because of King's vision, "we found the courage to come as far as we have."