OSLO, Norway - President Barack Obama accepted the Nobel Peace Prize with a striking defence of war, saying that evil must be vigorously opposed even as he made an impassioned case for building a "just and lasting peace."

"I face the world as it is, and cannot stand idle in the face of threats to the American people," Obama told his audience Thursday in Oslo's soaring City Hall as he was newly enshrined among the world's great peacemakers. "For make no mistake: Evil does exist in the world."

Pronouncing himself humbled by such an honour so early in "my labours on the world stage," Obama nevertheless turned his Nobel moment into an unapologetic defence of armed intervention in times of self defence or moral necessity. The hawkish message was an inevitable nod to the controversy defining his selection: an American president, lauded for peace just as he escalates the long, costly war in Afghanistan.

It was a jarring moment when Obama, in the midst of the ceremony, said of his troops in Afghanistan: "Some will kill. Some will be killed."

He lauded previous Nobel winner Martin Luther King Jr., as a preacher of nonviolent action. But he added, "A nonviolent movement could not have halted Hitler's armies. Negotiations cannot convince al-Qaida's leaders to lay down their arms."

"To say that force is sometimes necessary is not a call to cynicism, it is a recognition of history."

The president laid out circumstances in which war is justified - in self-defence, to come to the aid of an invaded nation, on humanitarian grounds such as when civilians are slaughtered by their own government.

At the same time, he also stressed a need to fight war according to "rules of conduct" that reject torture, the murder of innocents and other atrocities.

"We lose ourselves when we compromise the very ideals that we fight to defend," he said. "And we honour those ideals by upholding them not when it's easy, but when it is hard."

He emphasized a need to exhaust alternatives to violence, including worldwide sanctions with teeth to confront nations such as Iran or North Korea that defy international demands. He pushed himself away from George W. Bush in defending diplomatic outreach that engages even enemies. He defined peace as civil rights, free speech and economic opportunity, not just the absence of conflict.

"Let us reach for the world that ought to be," Obama said. "We can understand that there will be war, and still strive for peace."

Back in the U.S., presidential historians and foreign policy specialists saw the speech as underscoring Obama's revamping of America's stance - away from confrontation and toward co-operation and negotiation when possible, and military action when unavoidable.

Zbigniew Brzezinski, who was President Jimmy Carter's national security adviser, said Obama had presented "a very broadly stated case that we cannot in all circumstances avoid war." But he said he would have liked to have heard "some greater clarification of how he will pursue the broad objectives he has articulated."

The centerpiece of Obama's swift trip to Europe, the speech doubled the length of his inaugural address. Appearing tired here, Obama had worked all the way through the night on the flight to Norway, an aide said.

Such is the weight of he prize. Suddenly and forever, Obama is in the company of King, Nelson Mandela, Mother Teresa, and the Dalai Lama.

He drew laughter from his hosts when he acknowledged "the considerable controversy that your generous decision has generated."

"My accomplishments are slight," Obama said, by comparison to such "giants of history." In addition to other Nobel winners, he said the unrecognized masses who fight for peace are "far more deserving."

Obama's wife, first lady Michelle Obama, listened to her husband's words and showed tears by the end. The president was accompanied by a small crew of family, friends and advisers.

Politically, the White House was careful not to play up Obama's big award, with many families hurting economically at home and more troops heading off to war.

Earlier, the Obamas stepped onto their hotel balcony to wave to a crowd of thousands who had gathered for a torchlight procession. In the square below, there were chants of "Yes, we can" and "O-ba-ma" as scores of torches were held aloft. Nearby, up to 2,000 demonstrators protested, many carrying banners demanding the U.S. get out of Afghanistan.

Obama capped his evening with a sentimental toast at a candlelit dinner with Norwegian dignitaries, paying tribute to the influence of his late mother and the "largeness of her heart." And he spoke hopefully of the "extraordinary power" of the Nobel Prize to lift up those who might otherwise be forgotten.

On a damp and chilly day, the reaction on the streets of Oslo seemed curious but not overly excited. Crowds were not nearly of the size Obama has seen elsewhere. And there were protests.

The president will return to Europe next week to speak at the international conference on climate change, and activists pushing for a serious global climate deal challenged Obama to earn his award.

He won it, in the eyes of the Nobel committee, for changing the U.S. approach toward the world. The panel cited his efforts on nuclear disarmament, climate change and diplomacy.

"President Obama is a political leader who understands that even the mightiest are vulnerable when they stand alone," the committee chairman, Thorbjorn Jagland, said Thursday.

The Nobel comes with a $1.4 million prize. The White House says Obama will give that to charities but has not yet decided which ones.


Associated Press writers Matti Huuhtanen and Ian MacDougall contributed to this report.