Updated - includes videos - North Korea's long-time leader Kim Jong Il has died of heart failure.
In a "special broadcast" today from the North Korean capital, Pyongyang, state media said Kim died of a heart ailment on a train due to a "great mental and physical strain" on December 17, during a "high intensity field inspection".
The broadcast said an autopsy was carried out on December 18 and "fully confirmed" the diagnosis on the 69-year-old.
Kim is believed to have suffered a stroke in 2008, but he had appeared relatively vigorous in photos and video from recent trips to China and Russia and in numerous trips around the country carefully documented by state media.
The communist country's "Dear Leader" - reputed to have had a taste for cigars, cognac and gourmet cuisine - was believed to have had diabetes and heart disease.
"It is the biggest loss for the party ... and it is our people and nation's biggest sadness," an anchorwoman clad in black Korean traditional dress said in a voice choked with tears.
She said the nation must "change our sadness to strength and overcome our difficulties".
South Korean media, including Yonhap news agency, said Seoul had put its military on "high alert" and President Lee Myung-bak convened a national security council meeting after the news of Kim's death.
The news came as North Korea prepared for a hereditary succession. Kim Jong Il inherited power after his father, revered North Korean founder Kim Il Sung, died in 1994.
In September 2010, Kim Jong Il unveiled his third son, the twenty-something Kim Jong Un, as his successor, putting him in high-ranking posts. He was today being described as 'the great successor'.
Traffic in the North Korean capital was moving as usual today, but people in the streets were in tears as they learned the news of Kim's death.
Asian stock markets moved lower after the news, which raises the possibility of increased instability on the divided Korean peninsula.
South Korea's Kospi index was down 3.9% at 1,767.89 and Japan's Nikkei 225 index fell 0.8% to 8,331.00. Hong Kong's Hang Seng slipped 2% to 17,929.66 and the Shanghai Composite Index dropped 2% to 2,178.75.
North Korea said Kim's body will be placed in the Kumsusan memorial palace in Pyongyang and there will be a national mourning period until December 29. Kim's funeral will be held on December 28.