PARIS — The choice of Barack Obamaon Friday as the recipient of the 2009 Nobel Peace Prize, less thannine months into his eventful presidency, was an unexpected honor thatelicited praise and puzzlement around the globe.

Normally the prize has been presented, even controversially, foraccomplishment. This prize, to a 48-year-old freshman president, for “extraordinary efforts to strengthen international diplomacy and cooperation between peoples,” seemed a kind of prayer and encouragement by the Nobel committee for future endeavor and more consensual American leadership.

Butthe prize quickly loomed as a potential political liability — perhapsmore burden than glory — for Mr. Obama. Republicans contended that hehad won more for his star power and oratorical skills than for hisactual achievements, and even some Democrats privately questionedwhether he deserved it.

The Nobel committee’s embrace of Mr.Obama was viewed as a rejection of the unpopular tenure, in Europeespecially, of his predecessor, George W. Bush.

Butthe committee, based in Norway, stressed that it made its decisionbased on Mr. Obama’s actual efforts toward nuclear disarmament as wellas American engagement with the world relying more on diplomacy anddialogue.

“The question we have to ask is who has done the mostin the previous year to enhance peace in the world,” the Nobelcommittee chairman, Thorbjorn Jagland, said in Oslo after theannouncement. “And who has done more than Barack Obama?”

Still,Mr. Obama, who was described as “very surprised” when he received thenews, said he himself was not quite convinced, adding that the award“deeply humbled” him.

“To be honest,” the president said in the Rose Garden,“I do not feel that I deserve to be in the company of so many of thetransformative figures who have been honored by this prize, men andwomen who’ve inspired me and inspired the entire world through theircourageous pursuit of peace.”

He said, though, that he would“accept this award as a call to action, a call for all nations toconfront the challenges of the 21st century.” Mr. Obama plans to travelto Oslo to accept the award on Dec. 10. He will donate the prize moneyof $1.4 million to charity, the White House said.

Mr. Obama, only the third sitting American president to win the award, is suddenly put in the company of world leaders like Mikhail S. Gorbachev, who won for helping end the cold war, and Nelson Mandela, who sought an end to apartheid.

But less prominent figures have also won the award.

Thereaction inside the administration was one of restraint, perhapsreflecting the awkwardness of winning a major prize amid a worldwidedebate about whether it was deserved.

Republicans in Washington, reacting in disbelief, sought to portray Mr. Obama as unworthy. In an official statement, Michael Steele, chairman of the Republican National Committee, said, “The real question Americans are asking is, ‘What has President Obama actually accomplished?’ “

Butthere was much praise as well, even if Mr. Obama’s allies worried thatthe prize might be a liability and even if much of the praise came fromEurope, giving ammunition to conservatives who say Mr. Obama cares toomuch about opinion there.

President Nicolas Sarkozy of France said the award marked “America’s return to the hearts of the world’s peoples,” while Chancellor Angela Merkel of Germany said it was an “incentive to the president and to us all” to do more for peace.

“In a short time he has been able to set a new tone throughout the world and to create a readiness for dialogue,” she said.

Fora world that at times felt pushed around by a more unilateralist Bushadministration, the prize for Mr. Obama seemed wrapped in gratitude forhis willingness to listen and negotiate, as well as for his positionson climate change and nuclear disarmament.

Last year’s laureate, former President Martti Ahtisaari of Finland, saw the award as an endorsement of Mr. Obama’s goal of achieving Middle East peace.

“Of course, this puts pressure on Obama,” he said. “The world expects that he will also achieve something.”

The prize, announced as official Washington — including the president — was asleep, caught the White House off guard.

Thefirst word of it came in the form of an e-mail message to the WhiteHouse staff from the White House Situation Room, which monitors eventsworldwide around the clock, at 5:09 a.m. It carried the subject line“item of interest.”

Shortly before 6 a.m., the White House press secretary, Robert Gibbs, telephoned Mr. Obama, awakening him to share the news.

“There has been no discussion, nothing at all,” said the White House chief of staff, Rahm Emanuel.

The award comes at a time of considerable challenges for the president, with few sweeping achievements so far.

Onthe domestic front, he is pressing Congress to overhaul the nation’shealth care system. In foreign affairs, he is wrestling with hisadvisers over how to chart a new course in Afghanistan and has beenworking, with little movement, to restart peace talks between Israelisand Palestinians.

The Rose Garden appearance was an example ofMr. Obama’s heavy workload; it was squeezed into a day that alreadyincluded his regular intelligence and economic briefings, a privatemeeting with a senator, lunch with the vice president, a major speechoutlining plans for a new consumer protection agency and a strategysession on Afghanistan with his national security team.

Announcingthe award, the Nobel committee cited Mr. Obama “for his extraordinaryefforts to strengthen international diplomacy and cooperation betweenpeoples” and said that he had “created a new climate in internationalpolitics.”

In a four-paragraph statement, it praised Mr. Obamafor his tone, his preference for negotiation and multilateral diplomacyand his vision of a cooperative world of shared values, shorn of nuclear weapons.

“Onlyvery rarely has a person to the same extent as Obama captured theworld’s attention and given its people hope for a better future,” thecommittee said. “His diplomacy is founded in the concept that those whoare to lead the world must do so on the basis of values and attitudesthat are shared by the majority of the world’s population.”

The other sitting American presidents to be given the award were Theodore Roosevelt in 1906, for negotiating an end to a war between Russia and Japan, and Woodrow Wilson in 1919, for the Treaty of Versailles.

Former President Jimmy Carterwon in 2002 for his efforts over decades to spread peace anddevelopment. Mr. Carter called the award to Mr. Obama “a bold statementof international support for his vision and commitment.”

Former Vice President Al Gore won in 2007, sharing the prize with the Intergovernmental Panel on Climate Change, for his work on climate change. Mr. Gore called Mr. Obama’s award “well deserved” on Friday.

Mr.Obama has generated considerable goodwill overseas, with polls showinghim hugely popular, and he has made a series of speeches with archingambition. He has vowed to pursue a world without nuclear weapons;reached out to the Muslim world, delivering a major speech in Cairo inJune; and sought to restart peace talks between Israel and the Palestinians, at the expense of offending some of his Jewish supporters.

Buthe has had to devote a great deal of his time to the economic crisisand other domestic issues, and many of his policy efforts are onlybeginning.

In addition to the challenges in Afghanistan andPakistan, the situation in Iraq is extremely fragile; North Korea hasstaged missile tests; Iran continues to enrich uranium in defiance of United Nations Security Councilresolutions, though it recently agreed to restart nuclear talks; Israelhas resisted a settlement freeze; and Saudi Arabia has refused to makenew gestures toward the Israelis.

Ahmed Youssef, a Hamasspokesman, congratulated Mr. Obama but said the prize was based only ongood intentions. Muhammad al-Sharif, a politically independent Gazan,was incredulous. “Has Israel stopped building the settlements?” heasked. “Has Obama achieved a Palestinian state yet?”

The Nobelcommittee did not tell Mr. Obama in advance of the announcement, saidits chairman, Mr. Jagland. “Waking up a president in the middle of thenight,” he said, “this isn’t really something you do.”