Half a century ago, John Lewis, a 23-year-old student leader, stood on the steps of the Lincoln Memorial and looked out a sea of black and white faces. It was 1963, and the crowd had gathered in Washington for the most significant protest of the civil rights era.

Lewis saw hundreds of thousands of people, stretched into the distance; some had climbed trees for a better view, while others stood knee-deep in the memorial's long rectangular pool. "It was a hot day," Lewis recalls. "I said to myself: 'This is it, I must go forward.' And then I started speaking."

Now a 73-year-old Democratic congressman, Lewis is the only surviving speaker from the March on Washington, the landmark protest that culminated in Martin Luther King's famous "I have a dream" speech. He will return to the steps of the memorial later this month for the 50th anniversary of that march, which was a turning point for for the civil rights movement.

The March on Washington gave the campaign for equal rights an unstoppable momentum, helping to pass the Civil Rights Act and the Voting Rights Act – the two legislative pillars to emerge from the civil rights era. But according to Lewis, the 50th anniversary comes at difficult time for race relations in America. In an interview with the Guardian, he said:

The legacy of slavery and segregation dehumanises people. We have not yet escaped the bitterness. And we don't want to talk about it.

Two recent developments have jarred with the image of a country progressive enough to elect a black man to the White House. A recent Supreme Court decision effectively dismantled one of the key enforcement provisions in the Voting Rights Act, allowing southern states like Texas and North Carolina to implement changes to election rules than experts say discriminate against minority voters.

The later decision by a jury in Florida to acquit George Zimmerman over the killing of the black teenager Trayvon Martin has been cited by many – including president Barack Obama – as evidence of a legacy of persistent racial prejudice. "This is not a post-racial society," Lewis said. "Racism is still deeply embedded in American society, and you can't cover it up."

Lewis is a towering figure in the US, respected by both Democrats and Republicans. He is a cautious speaker, not known for controversial outbursts. But his remarks about the Zimmerman verdict are stark. "I don't think anything disturbed me more since the murder of Emmett Till," he said. "And I remember when Emmett Till was murdered, lynched, on 28 August 1955."

Till, a 14-year-old black boy from Chicago, was gruesomely murdered by racists while visiting family in Mississippi, after reportedly flirting with a white woman. Although the fatal shooting of Martin, who was 17, was a very different case, its aftermath stirred similar anguish in the country. Lewis, who represents a district in Georgia, said "a lot of pain" was caused by the case.

"I saw it in others, but I saw it also in myself," he said. "How can this happen in 2013, 50 years of the march on Washington, 150 years after the emancipation proclamation? How can it happen?"

'Between jails and luxurious salons of power'

John Lewis is ushered into police patrol wagon during a demonstration in Nashville, Tennessee in April 1964. Photo: Bettmann/Corbis

Part of the reason Lewis is revered today is because he was not only one of the "big six" civil rights leaders of the 60s, but a brave activist on the front line of often brutal encounters with segregationist authorities in the deep south. As chairman of the Student Nonviolent Coordinating Committee (SNCC) at the height of the civil rights movement, he was arrested more than 40 times and knew many of those who lost their lives fighting for the cause.

One day, he was leading marchers across the Edmund Pettus Bridge in Selma, where they were brutally attacked by Alabama State Troopers; the next he was in the Oval Office discussing the finer points of civil rights legislation. As Taylor Branch puts it in his brilliant book Parting the Waters, which documents the civil rights movement, Lewis was "shuttling between jails and luxurious salons of power".

When President Lyndon Johnson signed the Voting Rights Act into law, in 1965, he gave Lewis the pen he used.

Lewis recalls 1963 as an "extraordinary year" for America.

Segregation was real. You could feel it, and see it with your naked eye, all over the south. In parts of the deep south, you saw those signs that said 'whites' and 'coloureds'. There was a tremendous amount of fear in the states of the old Confederacy.

That year, president John F Kennedy had given a crucial speech, advocating an end to racial discrimination, and was pushing a civl rights bill through a Congress that was reluctant to embrace reform.

Lewis was in the White House when a fellow civil rights leader, A Philip Randolph, mentioned the idea of a mass mobilisation in Washington "for jobs and freedom". Kennedy, he remembers, was against the idea – fearing a breakout of violence that would strengthen the position of their mostly Republican opponents in Congress. But when the day finally arrived the enormous crowd, brought in on specially-chartered trains and buses from across America, upheld the tradition of peaceful dissent.

'The people literally carried us'

John Lewis speaks at the Lincoln Memorial. Photo: Bettmann/Corbis

Lewis recalls how he and the other speakers emerged from a Senate building, where they had been lobbying politicians, to find the march had started without them. They found their way to the front of the march, which by then had its own momentum. "The people literally carried us, they pushed us," he recalls.

The rally occurred at the Lincoln Memorial. The 23-year-old's speech, which criticised Kennedy's civil rights bill for being "too little, too late" and used the controversial word "revolution", was the most radical of any speaker. Lewis was forced to make minor textual changes at the last minute after white clergy, who were part of the broad coalition backing the march, objected to the content and threatened a boycott.

Despite the last-minute tweaks, his speech was said to have retained all of its authenticity. "I was speaking out of my own feeling, but I was also speaking out of the feeling and frustration of many of the people we had been working with in the south," he said.

I made it through the sit-ins. I had seen people sat at a lunch counter, and people pouring hot coffee on them and spitting on them.

Of course, it was the Baptist preacher who spoke after him who would be remembered most. King, who was assassinated five years later, was a spiritual leader for the civil rights movement. His widely remembered "I have a dream" passage was unplanned, and came after he ditched his written script.

The crowd was at least 250,000 strong – some estimates say that half a million people were there. But the power of the address rested in the fact it was beamed across America by radio and television networks. "By the time Martin Luther King Jr had finished his speech, he had preached a sermon to the American people," Lewis said. "He had shared a vision and he got other people to participate in that vision with him."

John Lewis sits to the left of Martin Luther King, at a Freedom Riders press conference in Montgomery, Alabama in May 1961. Photo: Bettmann/Corbis

Later this month, on 24 August, Lewis will speak again on the steps of the Lincoln Memorial, during a major recreation of the march and rally organised by Reverend Al Sharpton's National Action Network. It will not be as big as the 1963 protest, but organisers expect a substantial attendance – with buses packed with protesters coming from as far away as Minnesota and Miami.

The White House announced on Wednesday that Barack Obama will deliver remarks on the steps of the Lincoln Memorial on 28 August – the 50th anniversary of the march on Washington. Lewis will be there too. "I feel like I have an obligation to go there," Lewis said. "The people that I marched with, that I worked with, are all gone. Many of them are gone. So I have to represent them."