The five-member Norwegian Nobel Committee - four of whom spoke to The Associated Press, said awarding Obama the peace prize could be seen as an early vote of confidence intended to build global support for the policies of his young administration.
They lauded the change in global mood wrought by Obama's calls for peace and cooperation, and praised his pledges to reduce the world stock of nuclear arms, ease U.S. conflicts with Muslim nations and strengthen its role in combating climate change.
"Some people say - and I understand it - 'Isn't it premature? Too early?' Well, I'd say then that it could be too late to respond three years from now," Thorbjoern Jagland, chairman of the Norwegian Nobel Committee, told the AP. "It is now that we have the opportunity to respond - all of us."
Jagland said the committee whittled down a record pool of 205 nominations and had "several candidates until the last minute," but it became more obvious that "we couldn't get around these deep changes that are taking place" under Obama.
Obama said he was surprised and deeply humbled by the honor, and planned to travel to Oslo in December to accept the prize.
"Let me be clear: I do not view it as a recognition of my own accomplishments, but rather as an affirmation of American leadership on behalf of aspirations held by people in all nations," he said at the White House. "To be honest, I do not feel that I deserve to be in the company of so many of the transformative figures who've been honored by this prize."
Many were shocked by the unexpected choice so early in a presidency that began less than two weeks before the Feb. 1 nomination deadline for the prize and has yet to yield concrete achievements in peacemaking.
"So soon? Too early. He has no contribution so far. He is only beginning to act," said former Polish President Lech Walesa, who won the peace prize in 1983.
Aagot Valle, a lawmaker for the Socialist Left party who joined the Nobel committee this year, said she hoped the selection would be viewed as "support and a commitment for Obama."
"And I hope it will be an inspiration for all those that work with nuclear disarmament and disarmament," she told AP in a rare interview. Members of the committee usually speak only through its chairman.
The peace prize was created partly to encourage ongoing peace efforts, but Obama's efforts are at far earlier stages than those of past winners, and the committee acknowledged they may not bear fruit at all.
Republican Party Chairman Michael Steele said Obama won because of his "star power" rather than meaningful accomplishments.
"The real question Americans are asking is, 'What has President Obama actually accomplished?'" Steele said.
Drawing criticism from some on the left, Obama has been slow to bring troops home from Iraq and the real end of the U.S. military presence there won't come until at least 2012.
The Nobel committee said it paid special attention to Obama's vision of a nuclear-free world, laid out in a speech in Prague and in April and at the United Nations last month.
Unlike the other Nobel Prizes, which are awarded by Swedish institutions, the peace prize is given out by the five-member committee elected by the Norwegian Parliament. Like the Parliament, the panel has a leftist slant, with three members elected by left-of-center parties and two right-of-center members. Jagland said the decision to honor Obama was unanimous.
The secretive committee declined to say who nominated Obama. In Nobel tradition, nominations are kept secret for 50 years, unless those making the submissions go public about their picks.
This year's nominations included Colombian activist Piedad Cordoba, Afghan woman's rights activist Simi Samar and Denis Mukwege, a physician in war-torn Congo who opened a clinic to help rape victims.
Nominators for the prize are broad and include former laureates; current and former members of the committee and their staff; members of national governments and legislatures; university professors of law, theology, social sciences, history and philosophy; leaders of peace research and foreign affairs institutes; and members of international courts of law.
In his 1895 will, Alfred Nobel stipulated that the peace prize should go "to the person who shall have done (my emphasis) the most or the best work for fraternity between the nations and the abolition or reduction of standing armies and the formation and spreading of peace congresses."