TOKYO – Kim Jong Un, introduced on Monday as North Korea’s new leader, faces more formidable challenges than his father and grandfather, who ruled the country for a combined 63 years, as he assumes power in the impoverished nation without the stature and experience of his predecessors.

North Koreans learned Monday that Kim Jong Il, 69, had died of a heart attack. As they flooded into public squares, according to video released by Pyongyang’s news agency, many fell to their knees, sobbing, even howling. The government tried to reassure them of the readiness of Kim’s youngest son, declaring, “Under the leadership of Kim Jong Un, we should turn our sorrow into strength.”

Although he is hailed as the “Great Successor,” the new leader, thought to be in his late 20s, has neither the resume nor the skills needed to control the country in the rigid manner of his father and grandfather, experts say. And his father’s death has put him in charge long before he gained the allegiance of older officials.

The model that North Korea has clung to for six decades poses its own challenge: The country survives by controlling what its people say and do, harder to manage when the leader is young and untested, not a demigod.

Analysts who have studied North Korea’s second attempted power transfer, which began in September 2010, say they fear several scenarios, including a military revolt or a fight for power among older Workers’ Party members, who view Kim Jong Un as a vulnerable target.

Until late last year, most North Koreans had never seen a photograph of an adult Kim Jong Un. Pyongyang’s propaganda office had begun taking cautious steps to build the successor’s personality cult, particularly as Kim Jong Il, the “Dear Leader,” struggled with his health after a stroke in 2008. But the process was designed to last years, not months.

“This is really the worst possible nightmare for the North Korean state – this sudden death, and for the son to be taking over,” said Victor Cha, a former White House director of Asian affairs. “This could collapse before our eyes.”

Gauging the transition’s success could be difficult, particularly because Seoul and Washington have few ways to gain intelligence about the inner power circle in Pyongyang.

Behind the scenes, Kim Jong Il had spent years surrounding his heir apparent with those who were loyal to the Kim family and nobody else. He purged or banished senior officials who he considered power-hungry. He gave powerful positions to his sister, Kim Kyong Hui, and his brother-in-law, Jang Song Taek. If some high-profile officials are gone within the next few months, security experts said, it could be a sign of a fight for power that is threatening Kim Jong Un’s rise.

“Kim Jong Il tried to build a system where people owed their loyalty exclusively to him and his son,” said Ken Gause, an analyst who specializes in North Korean leadership. “But the idea that Kim Jong Un immediately starts making the decisions is a bit of a stretch. This is not a country that is used to collective leadership. That competition could eventually unravel, and that is one of the potential things that could cause instability.”

When Kim Jong Il formally took power from his father, Kim Il Sung, in 1994, he already had worked behind the scenes for almost two decades.

But the visible power transfer to Kim Jong Un began only 15 months ago, when North Korea held a massive political gathering in Pyongyang, naming him to several top military and Workers’ Party positions. Some outside experts fear that now, the nuclear-armed nation is more likely to carry out military attacks elsewhere in the region as a way to further burnish Kim Jong Un’s resume.

Some Seoul media outlets reported last year that Kim Jong Un was the mastermind behind a pair of 2010 attacks – the sinking of a South Korean warship and the shelling of a front-line South Korean island.

It is unclear how much support he has within the armed forces or the ruling party, both dominated by far older men.

For more than six decades, the Kim family has used North Korea as its own family-run business, gathering nuclear weapons, collecting luxury cars, paying little worry to chronic food shortages in the countryside and using isolation to hold it all together.

But Kim Jong Il’s death comes at a time when North Koreans have increased access to outside information. In an effort to bring in hard currency, the country has opened up to outside investors. With its central food-distribution system all but broken, North Korean officials have allowed for the emergence of private marketplaces – gathering spots where people can potentially share, in whispers, ideas they once kept to themselves.