Martin Luther King I have a dream speech
Martin Luther King, Jr., waves to supporters from the steps of the Lincoln Memorial on 28 August 1963 Getty

Fifty-one years ago today, Martin Luther King Jr, delivered a speech to over 250,000 civil rights supporters from the steps of the Lincoln Memorial, in a defining moment of the American Civil Rights Movement.

The March on Washington for Jobs and Freedom, or the Great March on Washington, was one of the largest political rallies for human rights in United States history.

The day after the march, King's "I Have a Dream" speech was carried live by television stations on 28 August 1963 - calling for civil and economic rights for African Americans and invoking the Declaration of Independence, the Emancipation Proclamation and the United States Constitution.

IBTimes UK looks at lesser-known facts about King's speech:

An expensive sound system installed for the event was sabotaged just before the speech. The Army Corps of Engineers was enlisted by Attorney General Robert Kennedy to fix it.

The phrase "I Have a Dream" is repeated eight times and the speech took just over 16 minutes to deliver. It is 1,666 words long.

Although the speech is called I Have a Dream, the words were not actually in the original draft but were ad-libbed on the day. Gospel singer Mahalia Jackson urged Dr King to tell the audience "about the dream" - and he went into an improvised section of the speech.

March on Washington
Martin Luther KIng (3rd from left) and other civil right leaders march on the Mall in Washington DC on 28 August 1963 Getty

The speech reportedly had several different titles and drafts in the beginning, including "The normalcy speech" and "A cancelled check".

William Edward Burghardt Du Bois, the co-founder of the National Association for the Advancement of Colored People (NAACP), died the day before the speech at the age of 95 in Ghana. Roy Wilkins asked the marchers to honor Du Bois with a moment of silence.

After stepping down from the podium, Dr King gave the typewritten speech to George Raveling, a Villanova basketball player who was acting as a volunteer guard. Raveling told CBS News: "I saw he had folded it up and I said, 'Dr King, can I have that speech?' He turned to hand it to me and appeared as if he was going to say something when a rabbi on the other side came up and congratulated him on what a wonderful speech it was. And that was the end of it."

March on Washington
An woman shouts "Freedom!" when asked to yell so loud it will be heard all over the world at the March on Washington Getty

Dr King did not have a solid idea for the speech 12 hours before, as he was so busy with the march, according to his co-authors.

Around 60,000 of the 250,000 people who attended the march were white - and people travelled from across the US to the event.

The marches had been proposed before President Kennedy's national address on civil rights and prior to the murder of NAACP state field director Medgar Evers in 1963. However, the events pushed Kennedy to approve the event - which he did in July when he was assured it would be peaceful.

Various sections of the speech were originally delivered as part of a speech Dr King gave in June 1963 after the Great Walk to Freedom in Detroit.