North Korea said Wednesday that millions of grief-stricken people turned out to mourn "Dear Leader" Kim Jong-Il, whose death has left the world scrambling for details about his young successor.

The North's propaganda machine has cranked into action to secure the late dictator's legacy and build up the same personality cult for his youngest son Kim Jong-Un, who inherits the world's last communist dynasty.

Television footage broadcast Wednesday showed tears streaming down Jong-Un's reddened face as he stood before his father's body.

It lies in state in a glass coffin at Pyongyang's Kumsusan Memorial Palace, surrounded by red Kimjongilia flowers named after the leader, whose death aged 69 was announced on Monday by a tearful television presenter.

The new ruler, clad in a black Mao suit, shook hands with distraught visitors in dark attire or military uniforms, occasionally bowing to them.

Beijing's ambassador to North Korea was among those who laid wreaths, China's official Xinhua news agency reported.

Elsewhere North Korean media showed mourners braving freezing conditions to pay their respects to Kim senior, seemingly overwhelmed by grief.

People in winter coats were shown weeping in a park blanketed with snow, laying flowers for the late leader, who presided over a 1990s famine that killed hundreds of thousands.

The North's official news agency reported at least five million people had visited statues and portraits around the capital Pyongyang to pay respects to the late leader -- more than a fifth of the entire population.

"These places turned into a veritable sea of mourners who bitterly wept, looking up to portraits of smiling Kim Jong-Il," it said.

Jong-Un's televised appearance provided a rare glimpse of the man poised to take the helm of the nuclear-armed nation while still in his late 20s.

Yonhap news agency quoted a senior Seoul government source as saying Jong-Un issued his first military order just before his father's death, commanding all units to halt field exercises and return to base.

"This is clear-cut evidence that Kim Jong-Un has secured a firm grip on the military," the official was quoted as saying.

North Korea's neighbours and the United States, which is treaty-bound to defend South Korea and Japan, are watching the transition warily.

Kim Jong-Un appears to have been accepted by the rest of the ruling elite as the official face of the new leadership, said Ralph Cossa, president of the foreign policy research institute Pacific Forum CSIS in Hawaii.

"They have as much a vested interest in a stable power transition as junior Kim does," he wrote in a commentary.

The senior Kim had reportedly been grooming his youngest son for the succession since suffering a stroke in August 2008. In September last year Jong-Un was appointed a four-star general and given senior party posts.

South Korea, still technically at war with the North, announced it would allow private groups to send condolence messages to its neighbour in another conciliatory gesture.

The move came a day after officials scrapped a plan to display Christmas lights near their shared border -- a proposal that had infuriated Pyongyang -- and sent its sympathies to the North Korean people.

Seoul had resumed the lights display last December, ending a suspension of several years, after a shelling attack by the North on a border island killed four South Koreans the previous month.

South Korea also accuses its neighbour of torpedoing a warship in March 2010 with the loss of 46 lives.

Defectors who fled Kim's harsh rule for South Korea launched tens of thousands of leaflets across the border by hot air balloon Wednesday, denouncing the late leader and calling for an Arab Spring-style uprising in the North.

"Rise up, people. Fight bravely like the Africans to end the third-generation succession," the leaflets read.

The head of a South Korean church that helps refugees from the North said recent signs of a military build-up along the communist state's border with China suggested fears of an exodus.

"I personally think there is a possibility of a mass defection," Pastor Kim Seung-Eun, of the Caleb Mission, told journalists in Seoul.