Tracing the footsteps of civil rights leaders who made his journey to the White House possible, President Barack Obama will on Saturday visit Selma, Alabama to enlist a new generation in their fight.

America's first black president will mark the 50th anniversary of the fateful march across the Edmund Pettus Bridge, where brutal state suppression only catalyzed support for black voting rights.

Accompanied by wife Michelle and daughters Sasha and Malia, Obama will argue that events in the city half a century ago are not confined to place or time.

"Selma is not just about commemorating the past," Obama said on the eve of the visit.

"It's about honoring the legends who helped change this country through your actions today, in the here and now."

On March 7, 1965, some 600 peaceful activists were attacked by police with clubs and tear gas at the bridge, a seminal moment in America's democracy.

A few months later, the Voting Rights Act was passed.

Before the act, it was easier for states like Alabama to restrict voter registration through violent intimidation and bureaucratic racism.

"Selma is about the courage of ordinary people doing extraordinary things because they believe they can change the country," Obama said.

"Selma is about each of us asking ourselves what we can do to make America better. And historically, it has been young people like you who helped lead that march."

The history of what happened at Selma on "Bloody Sunday" has recently returned to prominence thanks to an Oscar-nominated film starring actor David Oyelowo as Martin Luther King.

But Saturday's address also comes as today's rights leaders wrestle with scandals over police treatment of black Americans.

A probe by Obama's Justice Department unearthed deep police racism in Ferguson, Missouri, where a white policeman shot dead an unarmed black teenager on August 9, sparking civil unrest and a national outcry.

"I don't think that what happens in Ferguson is typical," Obama said Friday, but "it's not just a one-time thing."

Despite what Attorney General Eric Holder called a "searing report" into Ferguson's police department, the government will not prosecute the white policeman responsible for killing teenager Michael Brown.

Obama said he stood by the Justice Department decision.

"We may never know exactly what happened, but Officer (Darren) Wilson, like anybody else who is charged with a crime, benefits from due process and a reasonable doubt standard," said Obama.

"And if there is uncertainty about what happened, then you can't just charge him anyway just because what happened was tragic."