North Korean leader Kim Jong-il has died of a heart attack, state media has announced. He was 69.
His death has plunged the impoverished but nuclear-armed nation into uncertainty as it faces a second dynastic succession.
Kim's funeral will be held on 28 December in Pyongyang but no foreign delegations will be invited, the official Korean Central News Agency (KCNA) said.
A period of national mourning was declared from 17 to 29 December.
The leader "passed away from a great mental and physical strain" at 8.30am on Saturday (11.30pm Friday), while travelling by train on one of his field tours.
State TV urged people to follow his youngest son and heir apparent Kim Jong-Un, who is aged in his late 20s.
He was last year made a four-star general and given top ruling party posts despite having had no public profile.
"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," said a weeping black-clad TV announcer.
KCNA said Kim died of a "severe myocardial infarction along with a heart attack" and that an autopsy was performed yesterday.
The leader suffered a stroke in August 2008, which left him with impaired movement in his left arm and leg, and triggered an acceleration in the succession plans.
North Korea's propaganda machine has rolled into action to build up the same personality cult for Jong-Un that surrounded his father and late grandfather Kim Il-Sung, the founder and "eternal leader" of North Korea who died in 1994.
However, little is known about the succession. South Korea's top official on cross-border affairs said last month that there would be challenges in transferring power to the son.
The news sent shockwaves around the region where for years tension has run high over North Korea's nuclear ambitions and aggressive tactics.
South Korea has placed its troops and police force on emergency alert and summoned a meeting of the National Security Council.
President Lee Myung-Bak cancelled his schedule and called an emergency cabinet meeting.
The US Joint Chiefs of Staff said it had increased monitoring along the border along with US forces in the country but that no unusual activity had been observed.
"The President has been notified and we are in close touch with our allies in South Korea and Japan," said White House spokesman Jay Carney.
"We remain committed to stability on the Korean peninsula and to the freedom and security of our allies."
North and South Korea have remained technically at war since their three-year Korean conflict ended only in an armistice in 1953.
The news shocked South Koreans and some expressed fears of renewed conflict.
The European Union reacted cautiously to the news, saying it was "monitoring the situation closely".
Neighbouring China and Russia, both influential players in Pyongyang, sent their condolences and observers said Beijing would increase its all-important patronage to prevent an implosion in the communist North.
At the North Korean embassy in Beijing, the national flag was flying at half mast.
Japan, which has no diplomatic relations with North Korea, offered its condolences over the death.
US Senator John McCain voiced satisfaction that Kim is "in a warm corner of hell" and called for efforts to bring an end to his family's regime.
"The world is a better place now that Kim Jong-Il is no longer in it," Mr McCain said.
"I can only express satisfaction that the Dear Leader is joining the likes of Gaddafi, bin Laden, Hitler and Stalin in a warm corner of hell."