The Norwegian Nobel Committee praised the United Nations and its chief for working for peace throughout the world. It said Annan had brought "new life" to the world body and had fought for human rights and against AIDS and international terrorism.
Tributes rolled in from several trouble spots where Annan, 63, has brought his influence and diplomatic skills to bear.
In the Middle East, where Annan is one of the few U.N. officials to win Israeli as well as Palestinian acceptance as a man of good will, both communities endorsed the award.
Israeli Foreign Minister Shimon Peres praised Annan as a man with a humanitarian approach who "really worries about the poor of the world" and is "changing the face of the U.N.".
Palestinian negotiator Saeb Erekat welcomed the Nobel committee's choice, saying he hoped the world body would "gather the necessary strength" to end Israeli occupation and establish a Palestinian state with East Jerusalem as its capital.
Lebanese President Emile Lahoud, in a congratulatory telegram, thanked Annan for the role he and the United Nations had played in Israel's withdrawal from south Lebanon last year.
Portugal, the former colonial power in East Timor, lauded the efforts of Annan and the United Nations in bringing independence to the tiny East Asian territory.
"Truly, he was a key man so that the United Nations could play a different, constructive part in placing Timor on the road to independence," Portuguese Foreign Minister Jaime Gama said.
Nobel laureate and former South African President F.W. de Klerk praised Annan's "positive and constructive role in promoting peaceful solutions to conflicts around the world".
MASSACRE SURVIVORS
But Antoine Mugesara, a representative of Rwandan genocide survivors, recalled the "mess" he said Annan had made during the 1994 killings when he was head of U.N. peacekeeping operations.
"He has a heavy responsibility in the Rwandan genocide. It is a pity, it is unfortunate, he should not have been awarded that Nobel Prize," Mugesera told Reuters.
The Rwandan government, which was fiercely critical of Annan in the years after the genocide but has since patched up ties, extended qualified congratulations to the prize winners.
"We hope that the failures of the past will be a lesson for both the organisation and the secretary-general to make sure that peace and security is assured for all," said Joseph Mutaboba, permanent secretary in the Foreign Ministry.
Annan has apologised on behalf of the United Nations for its failure to prevent or halt the massacre of some 800,000 Rwandans, mainly ethnic Tutsis, by ethnic Hutu extremists.
In Bosnia, survivors of the 1995 massacre of up to 8,000 Muslim men and boys by Bosnian Serbs in the U.N. "safe area" of Srebrenica said they were "appalled" by the Nobel award.
"The United Nations and Kofi Annan are winners of the Nobel prize for genocide against the Bosniaks (Muslims) of the Drina valley and the whole of Bosnia," the Mothers of Srebrenica and Drina Valley association of survivors said in a statement.
Annan was head of U.N. peacekeeping at the time of the massacre. In 1999, he blamed the United Nations and key governments for failing to use force to prevent it.
WARM WORDS
European leaders were lavish in their praise.
"Today's announcement is marvellous news and richly deserved. No one and no organisation is more deserving," said British Prime Minister Tony Blair in a statement.
He said the award was an important signal as the West tries to bring to justice "those who struck at the heart of the free world just blocks from the U.N. headquarters", referring to the September 11 attacks on New York and Washington.
French Prime Minister Lionel Jospin hailed the award as a just recognition of Annan and of the United Nations as an institution needed more than ever in a troubled world.
"Via Kofi Annan and the United Nations, this Nobel Peace Prize lends remarkable support to all those who struggle for a world of peace, solidarity and justice," he said in a statement.
Words of approval also came from Austrian Chancellor Wolfgang Schuessel, Swedish Prime Minister Goran Persson and German Chancellor Gerhard Schroeder.
"If anyone has earned this award, taking into account his uncommon political achievement also in the present situation, then it is Kofi Annan," Schroeder told reporters.
U.N. High Commissioner for Human Rights Mary Robinson welcomed the Nobel award and compared U.N. workers in the field to the New York firefighters who responded to the last month's suicide-hijack attacks that killed nearly 5,600 people.
"I hope that the award of the Nobel prize to the United Nations will bring home that U.N. staff in the field are in the frontline of danger on behalf of humanity," she said.
U.N. High Commissioner for Refugees Ruud Lubbers voiced delight at the honouring of "a fabulous man".
Annan, who is the fifth African to win the Nobel Peace Prize, also won plaudits from envoys to the United Nations.
India's U.N. ambassador, Kamalesh Sharma, said the prize would make Annan a "real global celebrity, like a rock star".