US President Barack Obama has said he was "surprised and deeply humbled" to be awarded the Nobel Peace Prize, less than 10 months into his presidency.
Speaking at the White House hours after the Norwegian Nobel Prize Committee named him as a surprise winner, he said the award should be a "call to action".
The world faced challenges that "cannot be met by one person or by one nation alone," Mr Obama said.
The committee said he won for efforts to boost diplomacy and co-operation.
"Only very rarely has a person to the same extent as Obama captured the world's attention and given its people hope for a better future," the Norwegian committee said in a statement.
"His diplomacy is founded in the concept that those who are to lead the world must do so on the basis of values and attitudes that are shared by the majority of the world's population."
Standing in the Rose Garden to make his first public statement since being woken early by aides bringing news of the award, Mr Obama stressed that his win was just the beginning of his work.
He said he did not feel he deserved to be in the company of some of the "transformative figures" who had previously received the award.
Some of his aims, particularly the goal of universal nuclear disarmament, would be difficult to achieve even within his lifetime, let alone his presidency, Mr Obama said.
And he sought to deflect some of the global surprise at his win, describing the award as "affirmation of American leadership on behalf of aspirations held by people in all nations".
"I know that throughout history the Nobel Peace Prize has not just been used to honour specific achievements," he said.
"It's also been used as a means to give momentum to a set of causes. And that is why I will accept this award as a call to action, a call for all nations to confront the common challenges of the 21st Century."
The White House has said that the cash prize that accompanies the award will be distributed among several charities.
There were a record 205 nominations for this year's peace prize. Zimbabwean Prime Minister Morgan Tsvangirai and Chinese dissident Hu Jia had been among the favourites.
Instead the committee chose Mr Obama, who was inaugurated less than two weeks before the 1 February nomination deadline.
While there was support for the decision, notably from world leaders, many others expressed scepticism.
In the US the chairman of the Republican National Committee, Michael Steele, asked a simple question: "What has President Obama actually accomplished?"
Attributing Mr Obama's win to his "star power", Mr Steele said it was "unfortunate" he "outshined tireless advocates who have made real achievements working towards peace and human rights".
Iran's foreign minister said the decision to give the award was taken too "hastily".
"A good timing for the award would be when US troops have pulled out of Afghanistan and Iraq and the United States is standing up for the rights of the Palestinian people," Manouchehr Mottaki told the Mehr news agency.
But he said that if winning the prize encouraged the US president to reject the "warmongering" policies of previous administrations, Iran had no opposition to it.
A large majority of remarks from BBC viewers, listeners and website users also expressed surprise.
Senior Democratic figures rebuffed Mr Steele's remarks, with former Vice-President Al Gore, a joint recipient of the award in 2007, calling Mr Obama's win "extremely well deserved".
"I think that much of what he has accomplished already is going to be far more appreciated in the eyes of history," Mr Gore said.
But spokesmen from anti-US Islamist groups such as the Taliban and Hamas focussed on the present, saying they had seen no evidence yet of improvements in security for people in their regions and as such opposed the award.
Since taking office in January, President Obama has pursued an ambitious international agenda including a push for peace in the Middle East and negotiations over Iran's nuclear programme.
Asked why the prize had been awarded to Mr Obama less than a year after he took office, Nobel Committee head Thorbjoern Jagland said: "It was because we would like to support what he is trying to achieve".
He specifically mentioned Mr Obama's work to strengthen international institutions and work towards a world free of nuclear arms. The statement from the Nobel Committee said Mr Obama had "created a new climate in international politics".
However, critics say he has failed to make breakthroughs. Domestically, Mr Obama has been working to tackle an economic crisis and win support for healthcare reform.
Archbishop Desmond Tutu of South Africa, a former winner, said the prize was a way of encouraging the US leader early in his presidency.
Mr Obama is the first US president to win the prize since former US President Jimmy Carter in 2002. Theodore Roosevelt won the prize in 1906 and Woodrow Wilson won it in 1919.
The prize was invented by the Swedish industrialist and inventor of dynamite Alfred Nobel, and was first awarded in 1901.
As Sweden was at the time united with Norway, Nobel designated the parliament in Norway to elect the peace prize committee. Swedish academies are responsible for other prizes.
The prize-giving ceremony for the peace award is due to take place on 10 December in the Norwegian capital, Oslo. Mr Obama has indicated he will attend.