Barack Obama, bidding to become the first black President of the United States, scored a landslide win over his Democratic rival Hillary Clinton in the racially-charged South Carolina primary, blunting the momentum the former first lady enjoyed after her consecutive triumphs in New Hampshire and Nevada caucuses.

Riding on a massive African-American support, Obama routed New York Senator Clinton by 55 to 27 per cent in the state, where more than 50 per cent of the voters were black and four out of five supported him. Among white voters, Obama took only about a quarter of the vote, with Clinton and former Senator John Edwards roughly splitting the remainder.

Projecting himself as an agent of change, Obama said: "There are real differences between the candidates. We are looking for more than just a change of party in the White House. We are looking to fundamentally change the status quo in Washington."

"It's a status quo that extends beyond a particular party, and that status quo is fighting back with everything its got, with the same old tactics that divide and distract us from solving the problems people face," he said.

Obama, savouring his first victory after the Iowa caucuses, said his supporters had again proved the skeptics wrong.

"Over two weeks ago, we saw the people of Iowa proclaim that our time for change has come. But there were those who doubted this country's desire for something new, who said Iowa was a fluke not to be repeated again," he told a crowd of over 1,500 people.

"Well, tonight, the cynics who believed that what began in the snows of Iowa was just an illusion were told a different story by the good people of South Carolina," said Obama.

"The choice in this election is not about regions or religions or genders," the 46-year-old charismatic Democrat said in his victory speech.

"It's not about rich versus poor, young versus old and it's not about black versus white. It's about the past versus the future."

He declared that after four contests in every corner of US for party nomination for the Presidential elections, "we have the most votes, the most delegates and the most diverse coalition of Americans we've seen in a long, long time."

After her defeat, Clinton, who is also trying to script history by becoming the first woman President, said in a statement that she had called Obama to congratulate him.

"In the days ahead, I'll work to give voice to those who are working harder than ever to be heard," said the former first lady, who had emerged victorious in New Hampshire and Nevada primaries.

"For those who have lost their job or their home or their health care, I will focus on the solutions needed to move this country forward. That's what this election is about. It's about our country, our hopes and dreams. Our families and our future," she said in the statement.

Edwards came in third position with 18 per cent votes in the state where he was born and where he had scored a primary victory in his first presidential campaign four years ago. But he vowed to stay in the race.

"Our campaign from the very beginning has been about one central thing, and that is to give voice to millions of Americans who have absolutely no voice in this democracy," Edwards said.

"We're giving voice to all those Americans whose voices are not being heard, and their voices were heard today in South Carolina."

Obama, who also gained 25 convention delegates compared to Clinton's 12 and Edwards' eight, was widely anticipated to emerge victorious in the race.

But the nomination race remains wide open as South Carolina only set the stage for Super Duper Tuesday on February 5 when nearly two dozen states, including some of the big ones like New York, Illinois, California and Ohio, get to pick the candidates.