Throngs of somber, weeping mourners marched through the streets of the North Korean capital Monday, carrying flowers and offering prayers for Kim Jong Il, their reclusive leader whose death was announced earlier in the day.

Television footage from the closed communist state showed thousands of mourners in Pyongyang gathered at the foot of a giant statue of Mr. Kim, as the public sought to absorb the news of his death.

The official Korean Central News Agency announced early Monday that the 69-year-old Mr. Kim died of a heart attack Saturday while on a train for one of his “field guidance” tours. The agency attributed his death to “physical and mental overwork.”

The agency announced a state funeral for December 28 in Pyongyang, and declared a 10-day period of national mourning. The report said the government will not accept “foreign delegations hoping to express condolences,” and said no entertainment will be allowed during the national bereavement.

Late last year, Mr. Kim promoted his youngest son, Kim Jong Un, to the rank of four-star general, in what was seen as a bid to extend the world's only communist dynasty to a third generation.

State media urged citizens Monday to follow Kim Jong Un, who is believed to be around 28 years old. The possible succession of the inexperienced son is fueling speculation about the country's leadership transition.

Kim Jong Un's influence within the North Korean leadership is uncertain. Although he was appointed to several top posts last year, analysts say he has had little time to earn the trust of power brokers in the military and the ruling Korean Workers' party.

Washington has declined to make an assessment of the younger Kim. A White House spokesman Monday said the U.S. will judge the North Korean government as it always has, by its actions.

Kim Jong Il came to power after his father, North Korean founder Kim Il Sung, died in 1994. Since then, very little reliable biographical information about Mr. Kim has been available in the West. He rarely appeared in public and his voice was seldom broadcast.

He may be best remembered for defying the international community and boosting his country's nuclear program, while millions of North Koreans experienced widespread famine during much of the past two decades.