Women collapse in tears as North Koreans gather after learning of the death of their leader, Kim Jong Il, today in Pyongyang, North Korea. Kim died of heart failure Saturday at age 69.

PYONGYANG, North Korea — North Korea announced the death of supreme leader Kim Jong Il and urged its people to rally behind his young son and heir-apparent today, while the world watched warily for signs of instability in a nation pursuing nuclear weapons.

South Korea, anxious about the untested, 20-something Kim Jong Un after his father's 17-year rule, put its military on high alert against the North's 1.2 million-strong armed forces. President Barack Obama agreed by phone with South Korean President Lee Myung-bak to closely monitor developments.

People on the streets of the North Korean capital, Pyongyang, wailed in grief, some kneeling on the ground or bowing repeatedly as they learned the news that their “dear general” had died of heart failure Saturday at age 69 while carrying out official duties on a train trip.

“How could the heavens be so cruel? Please come back, general. We cannot believe you're gone,” Hong Son Ok shouted in an interview with North Korea's official television, her body shaking wildly.

“He passed away too suddenly to our profound regret,” said a statement carried by the North's official Korean Central News Agency. “The heart of Kim Jong Il stopped beating, but his noble and august name and benevolent image will always be remembered by our army and people.”

While there was no immediate statement on official succession, indications were clear that Kim Jong Un, the third son of Kim Jong Il, would be in charge.

The North said in a dispatch that the people and the military “have pledged to uphold the leadership of comrade Kim Jong Un” and called him a “great successor” of the country's revolutionary philosophy of juche, or self reliance.

The death could set back efforts by the United States and others to get Pyongyang to abandon its nuclear weapons ambitions, because the untested successor might seek to avoid any perceived weakness as he moves to consolidate control.

The death comes at a sensitive time for North Korea as it prepares for next year's 100th anniversary of the birth of founder Kim Il Sung — Kim Jong Il's father. The preparations include massive construction projects throughout the city as part of Kim Jong Il's unfulfilled promise to bring prosperity to his people.

Asian stock markets moved lower following the news.