The US is commemorating the 50th anniversary of the March for Jobs and Freedom, the civil rights rally at which Martin Luther King Jr made his “I have a dream” speech.
President Barack Obama is to mark the occasion in Washington DC with an address from the same spot.
Members of the King family and veterans of the march will also be present.
Mr Obama, the first black US president, has described the 1963 protest as a “seminal event” in American history.
The march was considered a catalyst for civil rights reforms in the US.
President Obama is due to deliver his address at the Lincoln Memorial on the National Mall just after an organised ringing of bells by churches and other groups at 15:00 local time (19:00 GMT), to mark the exact time that Martin Luther King spoke on 28 August 1963.
Mr Obama will be joined by former Presidents Bill Clinton and Jimmy Carter, as well as prominent African Americans.
Chat show host Oprah Winfrey and actors Forest Whitaker and Jamie Foxx led the celebrity contingent.
On Saturday, thousands of people, including King’s eldest son, marched to the Lincoln Memorial to mark the milestone anniversary.
Half a century earlier, Martin Luther King led some 250,000 protesters down the same strip and delivered his famous speech from its steps.
‘I have a dream’ revisited
“I have a dream that my four little children will one day live in a nation where they will not be judged by the colour of their skin but by the content of their character,” he said, in one of the most celebrated pieces of American oratory.
His address marked the peak of a series of protests against racial discrimination that had begun when seamstress Rosa Parks refused to give up her bus seat for a white passenger in 1955.
Her action sparked a bus boycott campaign across Montgomery, Alabama.
Marchers opened Wednesday’s damp commemoration by walking the streets of Washington DC behind a replica of the transit bus that Parks once rode.
King became a dominant force in the movement and so was called on to make the final speech at the march.
Many African Americans say King’s dream is still not a reality
He advocated the use of non-violent tactics such as sit-ins and protest marches, and was awarded the Nobel peace prize in 1964.
Four years later, his assassination led to rioting in more than 100 US cities.
Organisers of Wednesday’s commemoration are focusing beyond racial issues to address the environment, gay rights and the challenges faced by those with disabilities, among other matters.
In an interview on Tuesday with a radio show, President Obama said he imagines that King “would be amazed in many ways” about the social progress made since that speech.
He cited the prominent role of many African-Americans in the political and business spheres, as well as equal rights before the law.
Mr Obama, whose own oratory is often praised, said his address on Wednesday would not match that by the civil rights leader.
“It won’t be as good as the speech 50 years ago,” he said. “I just want to get that out there early.”
“When you are talking about Dr King’s speech at the March on Washington,” he added, “you’re talking about one of the maybe five greatest speeches in American history.”