At half-past one, Alabama time, on Sunday, thousands of people will make their way across the Pettus Bridge in Selma, marking the 50th Anniversary of Dr Martin Luther King's historic crossing in 1965 to demand voting rights for all.

On that occasion, the marchers were obstructed and attacked by troopers under the direction of the infamous sheriff Jim Clark, until President Lyndon B Johnston federalised the National Guard to enable the march to proceed. Even then three marchers died from attacks before the 54 miles to the State Capitol in Montgomery were completed.

Dr King then confronted the Governor, George Wallace, and this action was to secure the Voting Rights Act in Congress, a drama that is re-enacted in the newly released film Selma.

Fifty-one years ago, as a student spending a summer in the American South, I made a brief stop in Selma. Even, or perhaps especially, around the bus station, the atmosphere of fear and tension was evident. I had tried to make a symbolic point by sitting in the rear seats on the Trailways bus (it was accepted that the front seats were only to be occupied by whites, despite Rosa Park's courageous stand in Montgomery in 1956).

Despite that, I was momentarily caught unawares when the bus driver redirected me to a different door to enter the Selma bus station. I had come from Mississippi after spending some days with the student civil rights workers. Three had disappeared and were later to be found murdered by the local sheriff's henchmen.

I will accompany the Selma marchers on Sunday on the road to Montgomery, an event organised by Dr King's Southern Christian Leadership Conference. As we journey, I'll be questioning my fellow marchers about the changes over these years.

Dr King used to say that desegregation was useless without economic empowerment, and there is evidence that the income gap between whites and African Americans has widened during the Obama Presidency.

Although we might be policed on the march by a very different force than in 1965, recent events in the United States and figures about the racial mix in the prison population have highlighted the distance still to travel before policing is regarded in any positive way by the majority of African Americans.

My late friend David Batzka, veteran Civil and Human Rights campaigner, told me that he returned to Morgan Freeman's home town of Clarksdale, Mississippi on the 30th anniversary of his summer working there.

"As we came through the countryside," he told me, "I looked at the scenes in the cotton fields and they seemed no different from what we saw in 1964.'

The 1965 march was aimed at removing disenfranchisement that, apart from a brief period after the Civil War, had been the lot of African Americans since the birth of the Republic. For so much of that time, any rights were obliterated in the heinous system of slavery, whose legacy is all too evident today.

My host in Mississippi was Mr Albert Peters, a 95-year-old widower, born five years after the Emancipation Proclamation, who told me stories from his father's early life in slavery.

What I have noticed on various visits to the South for research and writing on anti-slavery, has been the way in which many places are publicly recognising their history in its entirety and not just selective parts. It's a lesson we are just beginning to learn in Scotland, not least with our involvement in slavery.

Generous friends in this country have sponsored my choice of charity as I walk to Montgomery to raise funds for the ongoing work of Anti-Slavery International today, and especially for the campaign to stop Human Trafficking, now one of the most profitable illegal industries in the world.

In every generation we need to reignite the struggle for human rights.