North Korean state media proclaimed his youngest son, Kim Jong Un, a "Great Successor," while a vigilant world watched for any signs of a turbulent transition to the untested leader in an unpredictable nation known to be pursuing nuclear weapons.
South Korea's military went on high alert in the face of the North's 1.2 million-strong armed forces following news of Kim's death after 17 years in power. North Korea said Kim died of a heart attack on Saturday while carrying out official duties on a train trip. President Barack Obama agreed by phone with South Korean President Lee Myung-bak to closely monitor developments.
On the streets of the North Korean capital, Pyongyang, people wailed in grief, some kneeling on the ground or bowing repeatedly. Children and adults laid flowers at key memorials.
A tearful Kim Yong Ho said Kim Jong Il had made people's lives happier. "That is what he was doing when he died: working, traveling on a train," he said.
Other North Koreans walked past a giant painting of Kim Jong Il and his late father, national founder Kim Il Sung, standing together on Mount Paektu, Kim Jong Il's official birthplace. Wreaths were neatly placed below the painting.
"How could the heavens be so cruel? Please come back, general. We cannot believe you're gone," Hong Son Ok shouted, her body shaking wildly during an interview with North Korea's official television.
A foreigner who teaches at a university in Pyongyang told The Associated Press that students told about Kim's death looked very serious but didn't show any outward emotion.
"There was a blanket of silence," said the teacher, who spoke on condition of anonymity because of worries about his security. "People were going about their business. Lots of people were lining up to lay flowers at official portraits. People looked a little stunned and very serious, but composed and respectful."
"He passed away too suddenly to our profound regret," said a statement carried by the North's official Korean Central News Agency. "The heart of Kim Jong Il stopped beating, but his noble and august name and benevolent image will always be remembered by our army and people."
He was 69, according to official records, though some reports indicate he was 70.
North Korean state media fell short of calling Kim Jong Un the country's next leader, but gave clear indications that Kim Jong Il's third son, who is believed to be in his late 20s, would succeed his father.
The North said in a dispatch that the people and the military "have pledged to uphold the leadership of comrade Kim Jong Un" and called him a "Great Successor" of the country's revolutionary philosophy of juche, or self reliance.
The death could set back efforts by the United States and others to get Pyongyang to abandon its nuclear weapons ambitions, because the untested successor may seek to avoid any perceived weakness as he moves to consolidate control.
"The situation could become extremely volatile. What the North Korean military does in the next 24-48 hours will be decisive," said Bill Richardson, a former U.S. ambassador to the United Nations who has made several high-profile visits to North Korea.
The death comes at a sensitive time for North Korea as it prepares for next year's 100th anniversary of the birth of Kim Il Sung. The preparations include massive construction projects throughout the city as part of Kim Jong Il's unfulfilled promise to bring prosperity to his people.
Seoul and Washington will worry that Kim Jong Un "may feel it necessary in the future to precipitate a crisis to prove his mettle to other senior leaders," said Bruce Klingner, an Asia analyst at The Heritage Foundation think tank in Washington.
North Korea conducted at least one short-range missile test Monday, a South Korean official said. South Korea's military sees the firing as part of a scheduled routine drill, instead of a provocation, said the official, speaking on condition of anonymity because of a policy that bans commenting on intelligence matters.
However, Konstantin Makienko of the Moscow-based Center for Analysis of Strategies and Technologies said the test "undoubtedly is connected to the death of North Korean leader Kim Jong Il."
"Its goal is to show the world that ... the armed forces of this country now are completely battle-ready and will react to any development," he told the Russian news agency RIA-Novosti.
North Korea conducted two nuclear tests in 2006 and 2009 and is thought to have enough plutonium for at least a half-dozen weapons. But experts doubt the North has mastered the technology needed to mount a nuclear warhead on a missile.
In Seoul, residents worried about instability in the North. A parliamentary official, Lee Kyu-yun. said he was thinking of stocking up food in case of soaring military tensions.
Lee Byung-joon, 27, feared South Korea might have to fight a war against the North if high-ranking officials challenge the inexperienced Kim Jong Un.
"I definitely think the chance of war breaking out between the South and the North is higher now than before," Lee said.
Some analysts, however, said Kim's death was unlikely to plunge the country into chaos because it already was preparing for a transition. Kim Jong Il indicated a year ago that Kim Jong Un would be his successor, putting him in high-ranking posts.
"There won't be any emergencies in the North, at least in the next few months," said Baek Seung-joo of the state-run Korea Institute for Defense Analyses in South Korea.
Another analyst said an internal power struggle could break out between Kim Jong Un and his uncle, Jang Song Thaek, who was elevated in the government last year and likely will be given a caretaker role in the new administration
"Tension will arise between Jang and Kim Jong Un, because Kim will have no choice but to share some power with Jang," said Ryoo Kihl-jae, a professor at the University of North Korean Studies, which is in South Korea.
The North said it would place Kim's body in the Kumsusan memorial palace in Pyongyang and that his funeral would be Dec. 28. No entertainment will be allowed during an 11-day mourning period, and the country will accept no "foreign delegations hoping to express condolences," it said.
South Korea's President Lee urged his people to remain calm while his Cabinet and the parliament convened emergency meetings. The Defense Ministry said the South Korean military and the 28,500 U.S. troops stationed in South Korea bolstered reconnaissance and were sharing intelligence on North Korea. Lee also talked with the leaders of Japan and Russia.
The Obama administration called Monday for a peaceful and stable leadership transition in North Korea.
The United States is still looking for better relations with the North Korean people despite the "evolving situation" there, said U.S. Secretary of State Hillary Rodham Clinton. "We both share a common interest in a peaceful and stable transition in North Korea as well as ensuring regional peace and stability," she said.
However, U.S. officials said Kim's passing and assumption of power of his son, Kim Jong Un, will likely delay anticipated developments on resuming nuclear disarmament talks with the North and supplying the nation with food aid.
The administration had been expected to decide on both issues this week. The officials said the U.S. was particularly concerned about any changes that Kim's death might spark in the military postures of North and South Korea, but were hopeful that calm would prevail.
In a special broadcast Monday from the North Korean capital, state media said Kim died on a train due to a "great mental and physical strain" during a "high intensity field inspection." It said an autopsy was done Sunday and "fully confirmed" the diagnosis. Kim suffered a stroke in 2008.
Associated Press writers Jean H. Lee in Pyongyang, Foster Klug, Hyung-jin Kim, Sam Kim and Jiyoung Won in Seoul and Matthew Lee in Washington contributed to this report.