On the night of Dr Martin Luther King Jr's assassination in Memphis, Tennessee, on 4 April 1968, New York Senator Robert F Kennedy, 42, was en route from Muncie, Indiana, to the state capital Indianapolis.

Kennedy was campaigning for the Indiana Democratic presidential primary, ultimately hoping to oust Lyndon Johnson from the White House and follow in the footsteps of his late brother John.

Hearing the news of the civil rights leader's murder on the balcony of the Lorraine Motel at the hands of white gunman James Earl Ray, Kennedy thought deeply about how he might break the news to the primarily black audience he was scheduled to address in an inner city park on 17th and Broadway.

He knew the audience would be devastated and was advised against appearing at all by the local police force who said they could not guarantee his safety in the event that violence broke out.

Kennedy's wife Ethel and his campaign staff likewise told him not to take the podium after angry residents were spotted nearby brandishing weapons and gas cannisters.

Rioting did indeed erupt across more than 100 American cities that night but Indianapolis would remain quiet.

Kennedy ignored his team and - wearing one of John's old overcoats - mounted the back of a flatbed truck to stand behind the microphone.

In his hands he clutched a small page of scribbled notes, going on to deliver one of the greatest speeches in American political history off the cuff, having turned down a proposed draft ghostwritten by an aide.

"I have some very sad news for all of you and, I think, sad news for all of our fellow citizens and people who love peace all over the world," he began.

"And that is that Martin Luther King was shot and was killed tonight in Memphis, Tennessee."

Audible screams arose from the crowd.

"Martin Luther King dedicated his life to love and to justice between fellow human beings. He died in the cause of that effort.

"In this difficult day, in this difficult time for the United States, it's perhaps well to ask what kind of a nation we are and what direction we want to move in."

Kennedy moved quickly and bravely to address the question of possible retaliatory violence head-on.

"For those of you who are black considering the evidence evidently is... that there were white people who were responsible, you can be filled with bitterness, and with hatred, and a desire for revenge.

"We can move in that direction as a country, in greater polarisation, black people amongst blacks and white amongst whites, filled with hatred toward one another.

"Or we can make an effort, as Martin Luther King did, to understand, and to comprehend, and replace that violence, that stain of bloodshed that has spread across our land, with an effort to understand, compassion, and love.

The candidate then invoked his own family tragedy, which had shocked the nation just four years before and was something he never publicly spoke about.

Doing so underlined the fact that, despite their superficial differences, he, a wealthy white Bostonian, was uniquely qualified to understand his audience's shock and grief.

"For those of you who are black and are tempted to fill with hatred and mistrust of the injustice of such an act, against all white people, I would only say that I can also feel in my own heart the same kind of feeling. I had a member of my family killed, but he was killed by a white man.

"But we have to make an effort in the United States. We have to make an effort to understand, to get beyond, or go beyond these rather difficult times."

Kennedy then quoted the Ancient Greek poet Aeschylus, a personal favourite:

Even in our sleep, pain which cannot forget

Falls drop by drop upon the heart,

Until, in our own despair,

Against our will,

Comes wisdom

Through the awful grace of God.

The reference was inspired. In the wrong hands, citing antiquity might have appeared pretentious or vainglorious but Kennedy respected his audience's intelligence and handed down the gift of a noble thought that had brought him much comfort in his own time of trial.

The speaker then looked ahead to how America could move forward from the shocking tragedy that had left the present looking so abject.

"What we need in the United States is not division; what we need in the United States is not hatred; what we need in the United States is not violence and lawlessness, but is love, and wisdom, and compassion towards one another, and a feeling of justice towards those who still suffer within our country, whether they be white or whether they be black.

"So I ask you tonight to return home, to say a prayer for the family of Martin Luther King - yeah, it's true - but more importantly to say a prayer for our own country, which all of us love a prayer for understanding and that compassion of which I spoke.

"We can do well in this country. We will have difficult times. We've had difficult times in the past; we will have difficult times in the future. It is not the end of violence; it is not the end of lawlessness; and it's not the end of disorder.

"But the vast majority of white people and the vast majority of black people in this country want to live together, want to improve the quality of our life, and want justice for all human beings that abide in our land."

Kennedy struck a sobering, realistic note, warning that racial harmony would not be achieved overnight or without further setbacks. But his message of hope was clear.

"Let's dedicate ourselves to what the Greeks wrote so many years ago: to tame the savageness of man and make gentle the life of this world.

"Let us dedicate ourselves to that, and say a prayer for our country and for our people."

Robert F Kennedy's speech was a masterpiece of oratory on one of the most difficult nights America had ever faced.

Martin Luther King: Life in pictures Show all 10 1 /10 Martin Luther King: Life in pictures Martin Luther King: Life in pictures Martin Luther King Martin Luther King during his famous 'I have a dream' speech in Washington in 1963 AP Martin Luther King: Life in pictures Martin Luther King Martin Luther King Jr at the Lincoln Memorial in 1963 AFP/Getty Martin Luther King: Life in pictures Martin Luther King American civil rights campaigner Martin Luther King (1929 - 1968) arriving at London Airport. He is in England to be the chief speaker at a public meeting about colour prejudice and to appear on the BBC television programme 'Face To Face' Getty Images Martin Luther King: Life in pictures Martin Luther King Martin Luther King Jr. tells a Miami, Florida news conference, that he will go to Los Angeles to meet with black and white leaders and help create "a community of love" in the violence torn city in August 1965 AP Martin Luther King: Life in pictures Martin Luther King Martin Luther King Jr. and his civil rights marchers head for Montgomery, the state's capitol, March 21, 1965, during a five day, 50 mile walk to protest voting laws AP Martin Luther King: Life in pictures Martin Luther King An image from 1960 shows Martin Luther King at a meeting Getty Images Martin Luther King: Life in pictures Martin Luther King Martin Luther King Jr. and his wife Coretta Scott King leading freedom marchers in Montgomery, Alabama in 1965 AP Martin Luther King: Life in pictures Martin Luther King Civil rights protestors marching from the Washington Monument to the Lincoln Memorial, where the March on Washington climaxed in Martin Luther King's 'I Have A Dream' speech Getty Images Martin Luther King: Life in pictures Martin Luther King Martin Luther King and the March on Washington BBC Martin Luther King: Life in pictures Martin Luther King March on Washington – 1963 Famous for Martin Luther King Jnr’s “I have a dream” speech, the march on Washington saw 300,000 people gathering at the Lincoln Memorial calling for equal rights for African-Americans Getty

As Georgia congressman and Selma marcher John Lewis observed: "That evening Robert Kennedy spoke from his soul."

A monument memorialises this graceful moment in the Indiana state capital today.

Kennedy would visit Dr King's widow Coretta at her home in Atlanta, Georgia, four days later to express his condolences in person.

But just 63 days after giving this historic address in Indianapolis, Bobby Kennedy himself was murdered in turn - the latest senseless killing to mar a decade of bloody turmoil.