Kim, who took over North Korea after his father and national founder Kim Il-sung died in 1994, "passed away from a great mental and physical strain" during a train ride at 8:30 a.m. on Saturday, the Korean Central News Agency said in an urgent dispatch.
KCNA later said the medical cause was a heart attack, the same ailment that killed his father.
A female newscaster, clad in a black funeral dress, tearfully announced Kim's death on state TV.
"He suffered an advanced acute myocardial infarction, complicated by serious heart shock, on a train on December 17 … from a great mental and physical strain caused by his uninterrupted field guidance tour for the building of a thriving nation," KCNA said.
The late Kim's health is believed to have worsened after he suffered an apparent stroke in 2008.
Kim's body will be placed in the Kumsusan Memorial Palace where the embalmed body of his father and North Korea founder Kim Il-sung lies, according to the KCNA. The North set a mourning period from Dec. 17 to Dec. 29, though it said it won't accept foreign delegations at the funeral ceremony scheduled to be held in Pyongyang on Dec. 28.
Kim's heir-apparent son, Jong-un, was named as chair of the North's funeral commission, state media said. The late leader had been grooming his youngest son as his successor, promoting him to the rank of four-star general and placing him in key posts at the ruling Workers Party.
South Korea put its military on high alert and ordered all of its diplomatic missions overseas to be on standby. The presidential office convened an emergency meeting of the National Security Council.
President Lee Myung-bak has called off all of his scheduled plans for later in the day.
The death occurred at a time when a flurry of diplomatic efforts are being made to resume long-stalled talks on ending Pyongyang's nuclear weapons program.