North Korea Thursday staged a massive memorial service for Kim Jong-Il to end almost two weeks of official mourning, and formally declared his untested young son Jong-Un as the new supreme leader.

Addressing tens of thousands of troops and civilians packing a wintry Pyongyang square, ceremonial head of state Kim Yong-Nam praised the late leader for contributing to "global peace and stability of the 21st century".

Kim Jong-Un, "the supreme leader of our party and army and people", had inherited his late father's spirit, leadership, personality, morality and fortitude, he added.

A sombre Jong-Un, clad in a black overcoat, presided from a balcony over the ceremony along with other members of the regime.

The service in Kim Il-Sung Square, named after the communist country's founder and father of the late Kim, concluded the mourning following Kim Jong-Il's death on December 17 from a heart attack at the age of 69.

The country observed three minutes of silence nationwide at noon (0300 GMT), punctuated by the horns of ships and railway engine whistles.

Analysts said the vast assembly was a show of confidence in Jong-Un, who inherits a daunting in-tray -- severe food shortages, a crumbling economy, acute power shortages and a nuclear programme which has alienated the West.

UN agencies have said six million people -- a quarter of the population -- urgently need food aid.

Whatever its internal difficulties, the North Korean regime presented a proud face to the outside world at the Pyongyang rally and further polished the cult of personality surrounding the Kim dynasty.

"The great heart of comrade Kim Jong-Il has ceased to beat... such an unexpected and early departure from us is the biggest and the most unimaginable loss to our party and the revolution," Kim Yong-Nam told the crowd, his voice throbbing with emotion.

The North would "transform the sorrow into strength and courage 1,000 times greater under the leadership of comrade Kim Jong-Un and will march firmly along the path of Songun taught by great leader Kim Jong-Il".

The Songun military-first policy prioritises the welfare of the 1.2 million-strong armed forces -- the world's fourth-largest -- over civilians.

Another speaker, Kim Jong-Gak, pledged the military's loyalty to the new leader, who has been named a general but has not served in the armed forces.

All troops "will become a wall to protect" him, the speaker said.

Next to Jong-Un on the balcony was military chief Ri Yong-Ho. Also in the line-up were Kim Jong-Gak, Kim Yong-Nam, senior ruling party officials Kim Ki-Man and Choe Thae-Bok, and defence minister Kim Yong-Chun.

The new leader's influential uncle Jang Song-Thaek was also on the rostrum, South Korea's unification ministry said. The service ended with an artillery salute.

"Today's event is an expression of confidence that the transition of power to Kim Jong-Un has been done smoothly," Kim Keun-Sik, of the Institute for Far Eastern Studies in Seoul, told AFP.

"It was also used to shore up loyalty to Jong-Un, showing North Koreans and outsiders that he has cemented his status as new leader."

China, North Korea's closest ally, said it was ready to work with the new regime to maintain friendly relations.

"We believe that the DPRK (North Korean) people will turn grief into strength under the leadership of the Worker's Party of Korea and Comrade Kim Jong-Un," a Chinese foreign ministry spokesman told reporters in Beijing.

In Japan, thousands of ethnic Koreans loyal to the regime flocked to memorial services in several major cities, including Tokyo, Nagoya and Osaka.

Jong Tae-Se, a striker in North Korea's World Cup squad of 2010, who was born in Japan and now plays for Germany's Bochum, was among those seen at a ceremony in the central city of Nagoya.

The memorial service followed Wednesday's huge funeral, which was seen as an attempt to bolster the image of Jong-Un -- who is in his late 20s and emerged from obscurity only in the past couple of years.

The son was at the forefront of the three-hour funeral procession which brought tens of thousands of shivering soldiers and civilians onto the snowbound streets of Pyongyang, many weeping or beating the frozen ground.

The state news agency said millions lined the 40-kilometre (25 mile) route of the funeral cortege.

Jong-Un walked bareheaded and gloveless alongside the hearse bearing his father's body for part of the way as it left and returned to the Kumsusan Memorial Palace.