Martin Luther King III was just 10 years old when his father was shot on the balcony outside room 306, at the Lorraine Motel in Memphis, on 4 April, 1968. Now 50 years old, he remembers the day vividly. "My siblings and I were watching the evening news at home in Atlanta and we heard 'Dr Martin Luther King Jr has been shot,'" says King III, who will lead a march tomorrow to that motel, now home to the National Civil Rights Museum.
"We ran to our mother's room, and she was preparing to leave because she had already had a call to say my father had been shot and she should get to Memphis as quickly as possible. She didn't know how bad the injury was."
Coretta Scott King never made it to Tennessee that night. An hour later, at 7.05pm, her husband was declared dead. "She came back to put us to bed, and told us that Daddy had gone home to live with God, and one day we'd see him again. She said that when we saw him again in his current form it would appear as if he were asleep; he wouldn't be able to talk to us or hug us or kiss us. But she told us that God rewards his servants and brings them home. That was enough for me at the time."
Only after his murder did King III begin to grasp his father's importance. He knew his father was friendly with some big stars – Harry Belafonte, Sidney Poitier, Sammy Davis – but it did not prepare him for the parade of celebrity mourners that descended on the family home in April 1968. "Every political aspirant who was running for office, came," he says. "Richard Nixon came. Robert Kennedy came. Ted Kennedy came. Jackie Kennedy Onassis came. Bill Cosby and Robert Culp, who starred together in I Spy and were one of the first racially integrated television pairings, came. Aretha Franklin, James Brown, Marlon Brando, Charlton Heston. All of the significant entertainers of the day. That's when I realised that my Dad must have been tremendously important."
On 9 April, the day of Dr King's funeral, President Johnson declared a national day of mourning. "I've never seen as many people in my life as I did on the streets of Atlanta that day," says King III. Until that day, he and his siblings had been largely sheltered from his father's public life. "To all of us, he was just Daddy," says King. "Because Mom did most of the disciplining, he was our playmate. When Daddy came home he would put us on the refrigerator and we would jump into his arms. We didn't have a large quantity of time with him, but the quality was remarkable. What I wish more than anything else is that as an adult I'd had the opportunity to have a conservation with him."
On rare occasions, King III and his brother Dexter travelled with their father. One trip he recalls clearly was to St Augustine, Florida, a town where the police were public servants by day and Ku Klux Klan members by night; when it was suggested that blacks should be allowed to use the public swimming pools, acid had been added to the water.
The four King children – King III has a younger brother and sister, in addition to an older sister, now deceased – also experienced discrimination first-hand. "There was a theme park in Atlanta called Fun Town," King remembers. "Whenever we drove past it we would always say we wanted to go in, and Daddy would say, 'Well, you can't'. One of the most incredible days we had together was the day when blacks were finally allowed to go to Fun Town. Daddy took us and we had a phenomenal time. Of course, right afterward we just forgot about it, but later I realised the significance of that day."
Martin Luther King III does not have Dr King's voice, nor his remarkable gift for oratory. Instead of the commanding rumble that inspired millions, his intonation is delicate. He does not have his father's striking face, nor his room-quieting charisma. Instead, he is fleshy and genial. But what he does have is his father's name. And 40 years after Martin Luther King Jr's death, it is a name that still carries great authority, and great responsibility.
"It weighs heavy on me every day," says King. "If I woke up each morning trying to fill my father's shoes, I would fail miserably. So I'm thankful my mother raised me and my siblings simply to be our best selves. 'Martin,' she said, 'you don't have to be Martin Luther King Jr.' I am who I am. All I can do is be the best Martin that I can be."
The young King III didn't harbour ambitions to follow his father into campaigning. "I was like any child. I wanted to be a pilot, a policeman, a bus driver, an astronaut. I worked in the hotel business for a few years after graduating from college, but I realised quickly that, though I enjoyed it, it was not my calling. I wanted to make a difference, not just make money."
But, from re-reading his father's sermons and speeches, King III learned much about the philosophy of non-violence. He cut his own teeth as a public speaker and organiser during his college years, and by co-ordinating the annual celebration of his father's January birthday – Martin Luther King Day.
In 2006 he founded Realizing the Dream, an organisation devoted to tackling poverty, one of the "triple evils" identified by Dr King before his murder. Forty years on, those triple evils of racism, poverty and militarism still loom large.
Dr King's best-known achievements – the 1964 Civil Rights Act and 1965 Voting Rights Act, specifically – tend to overshadow his later battles, fought in 1967 and 1968, against poverty and the Vietnam War. The latter position made him new enemies in the establishment, including J Edgar Hoover, the head of the FBI.
"We've made great strides race-wise, but we ignore the poor," says King III. "Hurricane Katrina showed the world there were Americans living in poverty. Yet even now our federal government acts as if poverty doesn't exist. If we'd spent some of the money we spent on the military in Afghanistan and Iraq on social services and business development instead, then maybe our economy wouldn't be so bad.
"We're still a long way from my father's dream of freedom and justice and equality for all."
Militarism, meanwhile, has run rampant. When it comes to King's views on the War on Terror, Dr King's principles of non-violence apply. "We had the world's attention after September 11th, the world's empathy and sensitivity," he says. "We squandered it. Everyone knew we could strike and knock people out, so why didn't we show the world that when you're attacked you don't always have to retaliate in kind? Maybe we need to reach out, try to understand and build relationships. That's leadership."
King III has high hopes that this year's presidential election can not only change things in America, but also in the wider world. He now supports Barack Obama but on Martin Luther King day in January, when every candidate was vying to lay claim to the King legacy, the great man's son was meeting not with Obama, but with his Democrat rival John Edwards. Edwards, he says, was the only candidate to have seriously raised the issue of America's vast underclass, the 37 million people now living in poverty. "Speaking up for them is not politically convenient," he wrote in a letter to Edwards. "But it is the right thing to do."
Since Edwards dropped out of the race, he has given his influential backing to Obama, the first credible black challenger to join the race for the White House. That a black man now has such an opportunity is one clear measure of Dr King's accomplishments. There have been black business leaders, black religious leaders and black political leaders who have inspired America's African-American community before now, but, says King III,
"Whether he is successful or not, the presence of Obama is going to create a different paradigm, a different model for leadership that tries to be more inclusive. The fact he's able to run as a serious candidate is, in itself, very significant."
The way the campaign has been conducted, however, has not always been to his liking. The media's focus on the fiery sermons of Reverend Jeremiah Wright, Obama's Chicago pastor is, he says, a sideshow. "That issue doesn't feed anyone," says King. "It doesn't help people get healthcare, it doesn't educate any children. We need to talk about race but we don't need to focus on Obama's relationship with a pastor. It's a decoy."
The television networks' penchant for re-running a passage of one sermon in which he cries "God Damn America!" has forced the pastor into hiding. "This, in the land of free speech," laments King III. "People say Reverend Wright hates America, and he hates white people. This is a man who has counselled biracial couples to get married in his own church. Now he can't go anywhere. The Klan said that if he came to Tampa, Florida, they would blow up the church."
King III was in London this week to attend an event, organised by the Arts Council-funded Music Matrix initiative, to commemorate his father. His speech is peppered with humour; he even quotes Richard Pryor.
Today's world is very different from his father's and, as the media reaction to Reverend Wright's sermon suggests, it is one in which his father's apocalyptic style would not necessarily be welcome.
"Reverend Wright's comments should create a dialogue. I don't agree with everything he said, but I've heard those sentiments from other black preachers all over the country. None of those people hate America. For example, someone still needs to explain how, in 12 or 15 years, all of Sub-Saharan Africa somehow contracted Aids. I don't know what created it but there's more to it than what's being said. I'm not sure it came through inoculations and vaccines, but I've heard a lot of preachers posit that theory as a possibility."
King III can be forgiven for believing conspiracy theories. The swirl of unanswered questions around his father's death produced plenty of those – not least a possible connection to Hoover's FBI. After involvement in the public investigation into Dr King's death, his brother drowned at home in an accident that his family still think suspicious.
King III's destiny, meanwhile, was written on his birth certificate. He and his wife are expecting their first child – and Dr King's first grandchild – in May. She may only share half of her grandfather's name, but while America retains its inequalities, it will remain an inspirational one, for at least one more generation.