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.
Pyongyang described his son Kim Jong-un as the "great successor" and urged North Koreans to unite behind him.
"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," North Korea's state-run news agency, KCNA, said.
Kim Jong-un, who is thought to be in his late 20s, was named as his father's successor just over a year ago.
A funeral for Kim Jong-il will be held in the capital Pyongyang on 28 December and Kim Jong-un will head the funeral committee, KCNA reports. A period of national mourning has been declared from 17 to 29 December.
The announcement of his death came in an emotional statement read out on national television.
The announcer, wearing black, wept as she said he had died of physical and mental over-work. A later report from KCNA said Mr Kim had had a heart attack.
Photos from Pyongyang showed people crying in the street as news of the death became known.
China - North Korea's closest ally and biggest trading partner - said it was "distressed" to hear the news of his death.
"We express our grief about this and extend our condolences to the people of North Korea," Foreign Ministry spokesman Ma Zhaoxu was quoted by Xinhua news agency as saying.
Amid concern that the news could lead to instability in the impoverished, nuclear-armed nation with few allies, South Korea's military was put on alert. Its National Security Council is convening 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.
South Korea's President Lee Myung-Bak spoke to US President Barack Obama by telephone.
"The two leaders agreed to closely co-operate and monitor the situation together," a South Korean presidential spokesman said.
Asian stock markets fell after the news was announced.
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 of 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, specialising in 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."
Christopher Hill, former US representative to the six-party talks on North Korea's nuclear programme, said all parties needed to "keep cool heads".
As for Mr Kim's son and successor, very little is known about him - including his exact age. He was educated in Switzerland and is the son of Mr Kim's reportedly favourite wife, the late Ko Yong-hui.
He has an older brother, Kim Jong-chol, and a older half-brother, Kim Jong-nam - both of whom appear to have been passed over for the succession.