Washington - President-elect Barack Obama faces a mammoth task in averting a series of domestic and global crises and managing the near-impossible expectations that have built up around his candidacy.

As congratulations poured in from leaders around the world Wednesday morning and Americans erupted in spontaneous celebrations, the tasks facing an Obama administration in January were clear.

Obama enters the White House with a shrinking US economy, a global financial system teetering on the brink of collapse and the United States still enmeshed in wars in Iraq and Afghanistan.

Already next week, Washington will host a summit of 20 world leaders to address the financial crisis that has sent global stock markets plummeting, and to establish common principles for future regulation of beleaguered financial sectors.

Obama himself acknowledged the "enormity" of the job ahead and asked for patience in his victory speech in Chicago.

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

Obama, 47, becomes the first-ever African-American president, and while his international popularity will give him some leeway at the start of his administration, the reactions Wednesday morning underlined the supremely high hopes for his presidency.

French President Nicolas Sarkozy said that Obama's victory "raises a great hope in France, in Europe and in the world - that of an open, united and strong America that will show a new way, with its partners, by the strength of its example and the adherence to its principles."

Palestinian President Mahmoud Abbas called on Obama to engage in the Middle East peace process "immediately, so as to bring security and stability to the Middle East region."

If saving the world economy and solving intractable conflicts aren't already an ambitious agenda, former South African president Nelson Mandela jotted another little chore on Obama's to-do list, urging the president-elect to "make it the mission of your presidency to combat the scourge of poverty and disease everywhere."

Obama has promised to work more closely with allies and signalled openness to direct talks with US adversaries. But he has also been unapologetic about pursuing US national security interests, for example, vowing to strike al-Qaeda targets inside Pakistan with or without Islamabad's approval.

At home, Obama has brought new voters into the electoral process with a message of hope and a promise of change from politics as usual, yet he faces a country that remains sharply divided along ideological lines.

Republican presidential candidate John McCain, who often derided Obama's lofty rhetoric as "empty words," was gracious in defeat and promised to help unite the country.

"I wish Godspeed to the man who was my former opponent and will be my next president," McCain said in Phoenix, Arizona.

African-Americans celebrated Obama's victory through the night in Washington and other cities and hope his election can help heal generations of racial division.

"It feels like anything is now possible," talk-show host Oprah Winfrey, an early Obama supporter, told broadcaster CNN. "The best is yet to come."

Amid his soaring oratory, Obama has often called on his admirers to be realistic in their expectations, and warned again in victory that there would be problems along the way.

"There will be setbacks and false starts. There are many who won't agree with every decision or policy I make as president," he said. "But I will always be honest with you about the challenges we face. I will listen to you, especially when we disagree."

The immediate challenge lies in the transition phase. President George W Bush's administration was in close contact with Obama's transition team well before Tuesday's election.

Obama's advisors are set to become involved immediately in policy making, and he will face pressure to quickly name key cabinet members for the Treasury, State and Defence departments.

The first measure of a seamless transition will be the G-20 summit on November 15 in Washington.

It was not immediately clear that Obama would attend. But with a lame-duck president at the helm, world leaders are sure to seek insights from the president-elect on the financial turmoil that will carry well into next year. (dpa)