US President Barack Obama rallied a new generation of Americans to the spirit of the civil rights struggle on Saturday, warning their march for freedom "is not yet finished."

In a forceful speech in Selma, Alabama on the 50th anniversary of the brutal repression of a peaceful protest, America's first black president denounced new attempts to restrict voting rights.

And he paid stirring tribute to the sacrifice of a generation of activists who marched so that black Americans could enjoy civil rights and opened the road that eventually led him to the White House.

"What a glorious task we are given, to continually try to improve this great nation of ours." —President Obama #Selma50 #MarchOn

— Barack Obama (@BarackObama) March 7, 2015

"We gather here to celebrate them," he declared, standing on the spot where Alabama state troopers launched an assault on the marchers in scenes that shocked America.

"We gather here to honor the courage of ordinary Americans willing to endure billy clubs and the chastening rod, tear gas and the trampling hoof, men and women who despite the gush of blood and splintered bone would stay true to their North Star and keep marching toward justice."

After the Selma march and others like it, then President Lyndon Johnson passed the Voting Rights Act that sought to prevent racist officials from excluding African Americans from the ballot.

That law, Obama said, is again under threat from state governments seeking to tighten voter registration rules in a bit to restrict the size of the franchise.

"How can that be?" he asked, noting that previous Republican presidents Ronald Reagan and George W. Bush -- who was present for the speech -- had renewed it.

"One hundred members of Congress have come here today to honor people who were willing to die for the right to protect it," he declared.

"If we want to honor this day, let those hundred go back to Washington and gather four hundred more and together pledge to make it their mission to restore the law this year.

"That is how we honor this bridge," he said, in front of a crowd estimated at 40,000, more than twice the population of what is still a very poor and mainly black town.

After the speech, Obama greeted civil rights veterans and he and Bush joined a 50-strong to group singing "Keep Your Eyes on the Prize" to walk across the Edmund Pettus Bridge -- named after a Confederate general and Klu Klux Klan leader.

"What an extraordinary honor this has been, especially to have Sasha and Malia here," Obama told the group, referring to his daughters, whom he has cited a members of the generation that must take the struggle forward.

The president and his wife, First Lady Michelle Obama, paused halfway across the road bridge to hear Georgia lawmaker and Selma marcher Congressman John Lewis describe the events of the day.

Honoring the legends

In his speech, Obama also addressed recent incidents in which police killings of unarmed black men and teenagers had triggered protests and accusations of deep-seated official racism.

"We just need to open our eyes and ears and hearts, to know that this nation's long racial history still casts its long shadow upon us. We know the march is not yet over, the race is not yet won," he said.

"We know that reaching that blessed destination where we are judged by the content of our character requires admitting as much, facing up to the truth."

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.

Obama was introduced by Lewis, a man he described as his hero, who as a young man was one of those assaulted on the bridge as he attempted to march.

"We come to Selma to be renewed, we come to be inspired," Lewis said.

"We were beaten, tear gassed, some of us were left bloody right here on this bridge. Seventeen of us were hospitalized that day. But we never became bitter or hostile."

Police racism

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 civil rights leader Martin Luther King.

But the president'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.

Despite what Attorney General Eric Holder called a "searing report" into Ferguson's police department, which Holder has not ruled out abolishing and replacing altogether.

On the eve of the Selma anniversary, protests broke out in the northern state of Wisconsin after another unarmed black teenager was shot by police, which said he had attacked an officer.

The police violence in Selma and the dignified determination of the marchers are widely seen as the factors that made it possible to swing America behind civil rights measures like the Voting Rights Act.

That legislation, which prevents states from limiting access to the ballot through abusive measures, is still challenged regularly, mainly by southern states seeking to make voter registration harder.