Watch Obama's election victory speech by John Whitesides

Barack Obama rode a wave of voter discontent to a historic White House victory, promising change as the first black US president but constrained by 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 voters emphatically rejected President George W. Bush's eight years of leadership.

The son of a black father from Kenya and white mother from Kansas, Obama was born at a time when black Americans were still battling segregationist policies in the South.

His election triumph over Republican rival John McCain on Tuesday is a milestone that could help the United States bury its long and often 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 more than 200,000 ecstatic supporters gathered in Chicago's Grant Park to celebrate.

Raucous street celebrations erupted across the country, but Obama will have little or no time off to enjoy the victory. He was expected to start work on Wednesday on planning a course for his formal takeover on Jan. 20 and putting together a team to tackle the huge challenges at home and abroad.

Obama won at least 349 Electoral College votes, far more than the 270 he needed. With results in from more than three-quarters of US precincts, he led McCain by 52 percent to 47 percent in the popular vote.

A first-term Illinois senator who will now be the 44th U.S. 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.

McCain's hopes for a surprise win evaporated with losses in a string of key battleground states led by the big prizes of Ohio and Florida, the states that sent Democrats to defeat in the last two elections.

McCain, a 72-year-old Arizona senator and former Vietnam War prisoner, 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 their windows to cheer.

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

"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 US Rep. John Lewis of Georgia, who was brutally beaten by police in Selma, Alabama, during a civil rights march in the 1960s.

Lewis was at a celebration in Ebenezer Baptist Church in Atlanta, the home church of Martin Luther King, who led the civil rights movement and was murdered in 1968.

Rev. Jesse Jackson, a prominent civil rights leader and former presidential candidate, joined the celebrations in Chicago on Tuesday night, tears streaming down his cheeks.

Obama 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 U.S. financial crisis, the worst since the Great Depression. Obama has proposed another stimulus package that could cost about $175 billion and include funding for infrastructure and another round of rebate checks.

Most world leaders welcomed Obama's victory.

Allied governments said they hoped for closer cooperation with Washington, while critics of the United States, ranging from officials in Russia and Iran to Islamist groups in the Middle East, called for clear changes in policy.

Obama took command of the election race in the last month as the financial crisis deepened and as his steady performance in three debates with McCain appeared to ease lingering doubts among voters.

His judgment on handling the economic crisis appeared to help tip the race in his favor. Exit polls showed six of every 10 voters listed the economy as the top issue.
In addition to Ohio and Florida, Obama won Virginia, Iowa, New Mexico, Nevada and Colorado - all states won by Bush in 2004. McCain's loss in Pennsylvania eliminated his best hope of capturing a Democratic-leaning state.
On the campaign trail, Obama had accused McCain of representing a third term for Bush's policies and being out of touch on the economy.

McCain attacked Obama as a tax-raising liberal and portrayed him as too inexperienced to run the country, but the Republican candidate struggled to distance himself from Bush. Exit polls showed three out of every four voters thought the United States was on the wrong track.

In the fight for Congress, Democrats gained at least five Senate seats and about 25 more House of Representatives seats, giving them a commanding majority in both chambers. (Reuters)