A special edition of a major South Korean newspaper Dong-A Ilbo is being printed to announce the death of North Korean leader Kim Jong-Il, in Seoul, on December 19.

"It was always a rumour before, not a confirmed report," she told AFP, referring to occasional rumours which surfaced in recent years.

A weeping black-clad announcer on the North's state television announced that the 69-year-old died of a heart attack while on a train during a field inspection visit.

The television and the official Korean Central News Agency urged people to follow Kim's youngest son and heir apparent Kim Jong-Un, who is aged in his late 20s.

"I'm worried there will be a war. I thought it wasn't true at first," said Song Bo-Na, 22, a university student.

Kim took over after his own father Kim Il-Sung died in 1994. Despite a stroke in August 2008, photos and TV footage had appeared to show him in better health in recent months.

"I hadn't much imagined what it would be like when Kim Jong-Il died, but now he has. I'm shocked and I'm very interested in what will happen from now on," said housewife Byun Mi-Sook.

Businessman Kim Sung-Il, 49, expressed expectations that North Korea would change after its leader's death.

"The death will be, and should be, the trigger for changes in and out of North Korea," he said.

Another businessman, Ko Jae-Lin, 50, also said he hopes for changes. "I think North Korea will now open up much sooner than speculated before," said Ko.

The North's regime had been resisting pressure from its ally China to free up the state-directed economy for fear of losing control.

Relations with the South's conservative government have been frosty since 2008.

University student Kim Lee-Soo, 21, said South Koreans and their government should "stay calm and try to respond appropriately, as this will have a huge influence in the South's stance in international society".

South Korea's government went on emergency footing after the shock news and summoned a meeting of the National Security Council.

North and South Korea have remained technically at war since the three-year Korean conflict ended only in an armistice in 1953.

"Monitoring and security around border areas has been strengthened. We are paying close attention to any movements by the North's military," a defence ministry spokesman told AFP.