Barack Obama rode a wave of voter discontent to an historic White House victory, promising change as the first black US president but facing enormous challenges from a deep economic crisis and two lingering wars.

Obama led Democrats to a sweeping victory that expanded their majorities in both houses of Congress as Americans emphatically rejected Republican President George W. Bush's eight years of leadership.

Democrats gained at least five Senate seats and about 25 seats in the House of Representatives, giving them a commanding majority in Congress and strengthening Obama's hand. Four Senate seats remained undecided.

The son of a black father from Kenya and white mother from Kansas, Obama was born when black Americans were still battling segregationist policies in the South. His triumph over Republican rival John McCain on Tuesday is a milestone that could help the United States get beyond its long, brutal history of racism.

"It's been a long time coming, but tonight, because of what we did on this day, at this defining moment, change has come to America," Obama, 47, told some 240,000 ecstatic supporters gathered in Chicago's Grant Park.

Bush fully embraced the election of Obama as his successor yesterday, paying stirring tribute to him.

Bush promised Obama his "complete cooperation" during the Democrat's 76-day transition to the White House. The president said he would keep Obama informed on all his decisions between now and January 20, and said he looked forward to the day that Obama and his family would take him up on his offer of a pre-inauguration White House visit.

He called Obama's win an "impressive victory", said it represented strides "toward a more perfect union" and said the choice of Obama was "a triumph of the American story, a testament to hard work, optimism and faith in the enduring promise of our nation".

Many world leaders welcomed Obama's victory and some hailed it as an opportunity to restore a tarnished US image.

Newspaper headlines captured the momentous nature of the result. A New York Times banner headline said simply "OBAMA", while the Washington Post declared "Obama Makes History" and USA Today: "America makes history; Obama wins".

Initial market reaction was muted. Analysts said Obama's victory had been largely priced in and concerns about the global economy were paramount.

On Wall Street, stocks were down in early trading, with the Dow Jones average dropping about 152 points, or 1.59 percent, to the 9,472 level. The dollar moved higher, recovering some of the previous session's heavy losses.

Obama won at least 349 Electoral College votes, based on state voting, far more than the 270 he needed. With 96 percent of the popular vote counted, he led McCain by 52 percent to 46 percent.

He will face intense pressure to deliver on his campaign promises. He has vowed to restore US leadership in the world by working closely with foreign allies, to withdraw US troops from Iraq in the first 16 months of his term and to bolster US troop levels in Afghanistan.

But his immediate task will be tackling the US financial crisis, the worst since the Great Depression.

A first-term senator from Illinois who will now be the 44th US president, Obama said he would work to ease the country's sharp political divisions and listen to those who voted against him.

"The road ahead will be long. Our climb will be steep. We may not get there in one year or even one term, but America, I have never been more hopeful than I am tonight that we will get there," he said in Chicago.

McCain called Obama to congratulate him and praised his inspirational and precedent-shattering campaign.

"I urge all Americans who supported me to join me in not just congratulating him but offering our next president our goodwill," McCain said.

Blacks and whites celebrated together in front of the White House to mark Obama's win and Bush's imminent departure. Cars jammed downtown Washington streets, with drivers honking their horns and leaning out of their windows to cheer.

Thousands more joined street celebrations in New York's Times Square and in cities and towns across the country.

"This is the most significant political event of my generation," said Brett Schneider, 23, who was in the crowd for Obama's victory speech in Chicago.

"This is a great night. This is an unbelievable night," said Republican John Lewis of Georgia, who was brutally beaten by police in Selma, Alabama, during a civil rights march in the 1960s.

Questions:

1. Why is Barack Obama’s win as the next President of the United States historic?

2. How long is the Democrats’ transition period to the White House?

3. What percent of the popular vote did Obama get compared to John McCain?

Answers:

1. He is the first black American to be president.

2. 76 days.

3. 52 percent over 46 percent.