Al Jazeera's Rosalind Jordan in Denver said Obama was also set to address the struggling US economy in the speech.

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Follow Obama's speech live on Al Jazeera's Twitter service 

Jordan said it was unclear how Obama would deal with the issue of race, with some US voters remaining sceptical about electing an African-American president.

In an unannounced appearance in the convention hall earlier on Thursday, Obama said he had moved the event to the centre's sports stadium as a tribute to his supporters.

"We want to open up the convention to make sure that everybody who wants to come can join in the party," he said, after appearing on stage following the acceptance speech of Joe Biden, his running-mate.

The televised speech is to take place on the 45th anniversary of Martin Luther King's "I have a dream" speech - a landmark in the US civil rights movement.

Martin Luther King III, the oldest son of the civil rights preacher, spoke in the run-up to Obama's speech.

In focus

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"We talk about freedom, justice and equality for all humankind, but when we look at the fact that healthcare is not available for all, our education system needs to be reformed, that people are losing their houses, and that the justice certainly for people of colour and poor people is not real yet," he told Al Jazeera after his speech.

"We cannot say we fulfilled his [Martin Luther King's] dream. We can say that yes, civil rights may take a new discussion.  But we still have some issues to overcome in relationship even to the issue of race."

Al Gore, the former vice-president, also spoke at the event, telling Democrats the US didn't need to recycle the policies of the Bush administration.

Marwan Bishara, Al Jazeera's senior political analyst, said Obama's speech would be aimed at middle-class voters as the Democrats could count on the votes of poorer Americans.

"He is trying to appeal to the middle-class, not necessarily to the working class, knowing that some 90 per cent of African-Americans will probably end up voting for him."

Republicans, who next week hold their own convention in St. Paul, Minnesota, to nominate John McCain, have said Obama is "unprepared" and say his soaring speeches mask a lack of substance.

Historic nomination

"The question for Obama is, 'What have you done and what have you run?'," Tim Pawlenty, the governor of Minnesota, named as a possible running-mate for McCain, told ABC news.

"He has good oratory but when you shut off the teleprompter there's not much there," he said.

Biden, a Delaware senator and chairman of the Senate Foreign Relations Committee, heaped praise on Obama during his acceptance speech on Wednesday.

Earlier, Obama and Biden had been officially selected as the US Democratic party candidates for November's presidential election after delegates at the party's convention approved his primary poll win.

The historic endorsement at the Denver convention hall made the Illinois senator the first African-American to receive the nomination of a major US party.

Obama's former rival, Hillary Clinton, formally asked delegates to suspend a roll call vote of states and approve his nomination by acclamation.