Denver, Colorado - Barack Obama became the country's first African-American to claim the presidential nomination of a major US party as Democrats Wednesday gave him a strong send-off into battle with presumptive Republican nominee John McCain.

Senator Obama, 47, the son of a white Kansas-born mother and a Kenyan father, was not expected until Thursday to appear at the convention and deliver his formal acceptance speech before a throng of 75,000 at Denver's Invesco Field.

But he strode into the convention hall at night's end, seizing the limelight from his rival Senator Hillary Clinton and former president Bill Clinton. They had commanded enthusiastic attention with their pro-Obama speeches Tuesday and Wednesday.

To all appearances, the party has achieved a major goal of healing wounds from one of its toughest nomination fights ever in a choice between two historic firsts - either a woman or an African-American.

"If I'm not mistaken, Hillary Clinton rocked the house last night," Obama said. "President Bill Clinton reminded us of what it's like when you have a president who actually puts people first."

The convention also nominated veteran Delaware Senator Joe Biden, 65, for vice president, who brings 35 years of Senate experience in foreign policy and national security to the Democratic ticket.

The long-time Delaware senator who has long held presidential ambitions spoke of his own working-class roots and checked off a list of "kitchen table" discussions US families were having during the troubled economic times.

Obama's campaign hopes those roots will also help convince blue- collar voters skeptical about Obama's race, his childhood in Hawaii and Indonesia and what his wife Michelle calls his "funny name."

Obama is locked in a dead-heat race against McCain, battling to stay ahead in the polls as Republicans open their nominating convention Monday in St Paul, Minnesota.

Obama and Biden are expected Friday to head by bus to the battleground state of Pennsylvania to launch their campaign.

Through the week, Democrats pounded McCain for his close association with US President George W Bush and blamed Republicans for America's eroded international standing, failure to take the lead against global warming and neglect of the country's pressing need for affordable health care and insurance.

Obama will likely set course on those issues in Thursday's acceptance speech.

Biden hammered McCain, a Vietnam war hero and former prisoner of war, for promising "more of the same" policies as Bush, for proposing 200 billion dollars in new tax breaks for corporate America but "no relief" for American families."

"The choice in this election is clear. These times require more than a good soldier - they require a wise leader. A leader who can deliver change," Biden said.

The nomination was a compelling piece of political theatre anchored in the traditional roll-call vote of the states despite some of Obama's advisors' concerns it could focus too much attention on Clinton.

Clinton tried to bring most of her large delegation - more than 40 per cent of the total - behind Obama, a gesture she needed to keep her high standing in the party and any chance of another White House bid down the road.

The task wasn't easy with some diehard supporters, whom she released just hours before to vote their conscience.

The process stopped after about 30 states when Obama had reached a a five-to-one lead over Clinton. That's when New York took the floor and passed the microphone to her.

"Let's declare together in one voice right here, right now that Barack Obama is our candidate," Clinton said, sending a surge of shouts and cheers rolling across the Pepsi Center convention hall. "I move Senator Barack Obama be selected by acclamation as nominee."

Obama was approved with a resounding cry of "aye" from more than 4,000 delegates. The delegation from Obama's native state of Hawaii waved American flags. The band struck up Love Train. Dancing and swaying erupted across the auditorium.

Later, former president Clinton threw his support fully behind Obama, saying the country needed his strong leadership to rebuild the American economic dream and "restore America's leadership in the world.

"Barack Obama is ready to lead America and restore American leadership in the world," Bill Clinton said. "He will choose diplomacy first and military force as a last resort."

Despite the Clinton family's eagerness to seal party unity, Obama's campaign is wary of how to deploy the former president in general election campaign after being at the wrong end of Clinton's sharp tongue during the hard-fought primary elections. (dpa)