The 2005 Nobel peace prize has been awarded jointly to the United Nations' International Atomic Energy Agency and its director, Mohamed ElBaradei.
The citation says the IAEA's director is a "fearless advocate" of curbing nuclear arms and the importance of his agency's work "incalculable".
The nuclear watchdog's work to promote safe nuclear energy was also commended.
Mr ElBaradei said the award recognised that the spread of nuclear weapons was the world's worst security threat.
Speaking at the IAEA headquarters in Vienna, he said he had learnt of the decision at home while watching the announcement live on TV along with his wife.
They were taken completely by surprise, he said, as they had not been contacted by telephone and assumed the IAEA had not won.
"I'm extremely humbled and honoured," the Egyptian former diplomat said.
"I think the prize recognises the number one danger we are facing today and that is the threat of proliferation of nuclear weapons."
He began a third term at the IAEA last month after the US withdrew its objections to his reappointment. Washington had complained that he was being "soft" on Iran.
'Policeman and salesman' IAEA inspectors have had to deal with major crises in Iraq, North Korea and Iran in recent years.
Over the past couple of years, the 63-year-old IAEA director has also overseen investigations into the nuclear black market led by Pakistani scientist AQ Khan.
Many analysts hail the work of the agency, saying it plays a key role around the world in ensuring that nuclear material is not used for making weapons.
The BBC News website's Paul Reynolds notes that Mr ElBaradei's style is very much that of the technocrat who makes his impact with his expert analysis.
In negotiations with Iran, he has shown that he cannot be fooled by bogus technical arguments, while remaining a fair judge.
But critics say the IAEA does not deserve the prize, accusing it of failing, for example, to prove conclusively whether or not Iran's nuclear ambitions are peaceful.
And opponents of nuclear civilian energy condemn the agency's commitment to atomic power.
Greenpeace International spokesman Mike Townsley said Mr ElBaradei was trapped by the agency's "contradictory role, as nuclear policeman and nuclear salesman".
'Very disappointing' This year's contest saw 199 names put forward - a record number.
Leaks ahead of the 2003 and 2004 prize announcements led the Nobel Committee to tighten secrecy this year.
News of the decision provoked mixed feelings among rivals for this year's peace prize.
"It's an excellent choice... for an organisation that does extremely good work," said Finnish President Martti Ahtisaari, strongly tipped as a 2005 winner for his peace work in Indonesia.
But Hidankyo, a Japanese association representing survivors of the 1945 atomic bomb attacks, said it was "very disappointing".
"We had hoped that if we were able to win the award, it would give us a big boost and provide great encouragement for hibakusha [atomic bomb victims] who are growing old," said Terumi Tanaka, Hidankyo's secretary general.
The IAEA, he added, was "not an organisation that has conducted peace activities".
The annual international peace prize, worth $1.29m (?725,000), has been awarded since 1901, after its establishment by Swedish industrialist and inventor Alfred Nobel.
In keeping with tradition, it will be awarded in the Norwegian capital while the other Nobel prizes - for excellence in physics, chemistry, medicine and literature - are awarded in Stockholm.
http://news.bbc.co.uk
I didn't even know our former president was a favorite to win.....
/jarmo
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