MISSISSIPPI - REPUBLICAN John McCain and Democrat Barack Obama on Friday clashed in a crucial first presidential debate with the United States reeling from days of deep political and economic turmoil.

Mr Obama, 47, vying to make history as America's first black president, looked directly into the camera, addressing tens of millions Americans watching on television, hoping to win over remaining doubts about his capacity to lead.

The Illinois senator, who has pulled off an astonishingly quick rise to the pinnacle of US politics, took the first question, on the economic crisis wracking Wall Street which has infected global markets.

'We are at a defining moment in our history,' said Mr Obama, who wore a dark blue suit, white shirt and red tie.

Republican candidate John McCain, the 72-year-old Vietnam war veteran and long-time senator only decided 10 hours before the fiercely anticipated debate that he would show up, after saying it should be delayed by the financial crisis.

He warned that a grave economic crisis was at hand, and said he was glad that finally the two parties were negotiating on a rescue package.

Democrats have accused Mr McCain of derailing a rescue package with a sudden return to Washington to inject himself into the negotiations over a US$700b (S$999b) bailout package.

But the Arizona senator, wearing a red and white tie and a dark blue suit, said if it had not been for him, recalcitrant Republicans needed to pass the deal through Congress would have been left out of the tent.

'This isn't the beginning of the end of this crisis, this is the end of the beginning,' he said, paraphrasing one of his heroes, wartime British prime minister Winston Churchill.

The candidates stood at wooden lecterns at a slight angle to each other in front of the audience and the debate's moderator, veteran PBS anchor Jim Lehrer.

'We are all witnessing history,' Mr Lehrer told the audience shortly before the start, warning against interruptions from applause or boos.

'Remember this is for those 270 million people who are about to make a decision about who is going to run this country,' he said.

Mr McCain had kept everyone, including his staff, guessing about whether he would attend, after dramatically suspending his campaign on Wednesday to rush back to Washington for talks on Wall Street's financial meltdown.

But his campaign finally said earlier on Friday that enough progress had been made in the talks in Congress on a US$700b bailout that the senator would fly to Oxford for the 90-minute debate, due to focus on foreign policy.

Insights from advisers

Asked what Mr McCain hoped to achieve in the debate, one of his top advisers Mark Salter said: 'To do well against a guy who is a pretty good debater, show presidential leadership, and be able to speak directly to the American people about what he believes.'

Obama spokesman Bill Burton vied to ramp up expectations for Mr McCain, a national security expert.

'If he slips up, makes a mistake, or fails to deliver a game-changing performance, it will be a serious blow to his campaign,' Mr Burton said.

'Given his unsteady performance this week, he desperately needs to win this debate in a big way in order to change the topic and get back to his home turf.'

Presidential debates have often played a crucial role in the outcome of elections.

In 2004, more than 62 million people watched the first debate between President George W. Bush and Mr John Kerry.

Given the historic potential of this year's gripping election race, analysts expect an even bigger audience.

And they said a strong performance by either candidate in the debates, or a string of gaffes, still have the potential to remake the race in the crucial final stretch up to the vote. -- AFP