The speech was being given in an outdoor stadium in a bid to show his candidacy extends beyond the politicians who have dominated the convention so far, his aides said.

Mr Obama was delivering what has been labelled the most important oration of his life, almost exactly 45 years after Martin Luther King Jr inspired the world with his "I have a dream" speech.

But after being labelled a celebrity who is not ready to be America's commander-in-chief by his Republican rival John McCain, Mr Obama's campaign chiefs tried to make the occasion feel more intimate and personal than the boisterous rallies which, in one instance, attracted more than 200,000 people.

Camera angles were due to show him among the crowd, rather than above it, and efforts were being made to project Mr Obama as an ordinary American who understands the problems faced by voters.

He was to address America, and the world, from a stage featuring faux columns which look like a Greek temple – designed to suggest the gravitas of a federal building in Washington DC. It has immediately been labelled the "Temple of Obama" by Republicans.

Many US political pundits agreed that the 47-year-old Illinois senator would have to deliver more than a lofty, inspirational speech with soaring rhetoric. Instead, he was expected to focus on America's troubled economy, a key issue in the election, and also come out on the offensive against Mr McCain.

He told the Wall Street Journal: "The single most important thing I have to make clear is the choice we have in November between the same failed policy of the last eight years for the middle class and the new agenda to boost income for Americans and help families who are struggling."

Mr Obama's campaign manager, David Plouffe, told ABC's Good Morning America: "I think what Senator Obama wants to do is make sure everyone watching at home is going to have a clear sense of where he wants to take the country."