Spoiler: show
North Korean leader Kim Jong-il has died at the age of 69, state-run television has announced.
Mr Kim, who has led the communist nation since the death of his father in 1994, died on a train while visiting an area outside the capital, the announcement said.
He suffered a stroke in 2008 and was absent from public view for months.
His designated successor is believed to be his third son, Kim Jong-un, who is thought to be in his late 20s.
North Korea's state-run news agency, KCNA, urged people to unite behind the younger Kim.
"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-un and protect and further strengthen the unified front of the party, military and the public," the news agency said.
A funeral for Kim Jong-il will be held in Pyongyang on 28 December and Kim Jong-un will head the funeral committee, KCNA said. A period of national mourning has been declared from 17 to 29 December.
Heart attack
The BBC's Lucy Williamson in Seoul says Mr Kim's death will cause huge shock waves across North Korea, an impoverished, nuclear-armed nation with few allies.
The announcement came in an emotional statement read out on national television.
The announcer, wearing black, said he had died of physical and mental over-work. A later report from KCNA said Mr Kim had had a heart attack.
South Korea's military has been put on alert following the announcement and its National Security Council is convening for an emergency meeting, Yonhap news agency reports. The Japanese government has also convened a special security meeting.
The White House said it was "closely monitoring" reports of the death. The US remained "committed to stability on the Korean peninsula, and to the freedom and security of our allies", it said in a statement.
Asian stock markets fell after the news was announced.
Isolated regime
Mr Kim inherited the leadership of North Korea - which remains technically at war with South Korea - from his father Kim Il-sung.
Shortly after he came to power, a severe famine caused by ill-judged economic reforms and poor harvests left an estimated two million people dead.
His regime has been harshly criticised for human rights abuses and is internationally isolated because if its pursuit of nuclear weapons.
Under Mr Kim's leadership funds have been channelled to the military and in 2006 North Korea conducted its first nuclear test. It followed that up with a second one three years later. Multinational talks aimed at disarming North Korea have been deadlocked for months.
Mr Kim unveiled his son as his likely successor a year ago. Many had expected to see this process further consolidated in 2012.
Professor Lee Jung-hoon, professor of international relations at Yonsei University in Seoul, told the BBC that with the transition of power from father to son incomplete, Mr Kim's death could herald "very unstable times" in North Korea.
"We have to be very worried because whenever there is domestic instability North Korea likes to find an external situation to divert the attention away from that - including indulging in provocation."
Mr Kim, who has led the communist nation since the death of his father in 1994, died on a train while visiting an area outside the capital, the announcement said.
He suffered a stroke in 2008 and was absent from public view for months.
His designated successor is believed to be his third son, Kim Jong-un, who is thought to be in his late 20s.
North Korea's state-run news agency, KCNA, urged people to unite behind the younger Kim.
"All party members, military men and the public should faithfully follow the leadership of comrade Kim Jong-un and protect and further strengthen the unified front of the party, military and the public," the news agency said.
A funeral for Kim Jong-il will be held in Pyongyang on 28 December and Kim Jong-un will head the funeral committee, KCNA said. A period of national mourning has been declared from 17 to 29 December.
Heart attack
The BBC's Lucy Williamson in Seoul says Mr Kim's death will cause huge shock waves across North Korea, an impoverished, nuclear-armed nation with few allies.
The announcement came in an emotional statement read out on national television.
The announcer, wearing black, said he had died of physical and mental over-work. A later report from KCNA said Mr Kim had had a heart attack.
South Korea's military has been put on alert following the announcement and its National Security Council is convening for an emergency meeting, Yonhap news agency reports. The Japanese government has also convened a special security meeting.
The White House said it was "closely monitoring" reports of the death. The US remained "committed to stability on the Korean peninsula, and to the freedom and security of our allies", it said in a statement.
Asian stock markets fell after the news was announced.
Isolated regime
Mr Kim inherited the leadership of North Korea - which remains technically at war with South Korea - from his father Kim Il-sung.
Shortly after he came to power, a severe famine caused by ill-judged economic reforms and poor harvests left an estimated two million people dead.
His regime has been harshly criticised for human rights abuses and is internationally isolated because if its pursuit of nuclear weapons.
Under Mr Kim's leadership funds have been channelled to the military and in 2006 North Korea conducted its first nuclear test. It followed that up with a second one three years later. Multinational talks aimed at disarming North Korea have been deadlocked for months.
Mr Kim unveiled his son as his likely successor a year ago. Many had expected to see this process further consolidated in 2012.
Professor Lee Jung-hoon, professor of international relations at Yonsei University in Seoul, told the BBC that with the transition of power from father to son incomplete, Mr Kim's death could herald "very unstable times" in North Korea.
"We have to be very worried because whenever there is domestic instability North Korea likes to find an external situation to divert the attention away from that - including indulging in provocation."
