North Korea on Monday mourned the death one year ago of leader Kim Jong-Il, with its rocket scientists taking pride of place at a special memorial ceremony led by his son and successor Kim Jong-Un.

Kim Jong-Il died of a heart attack on December 17, 2011, although his death was only announced two days later.

Monday's ceremony, attended by hundreds of top party and military cadres, was held in the Kumsusan Palace of the Sun in Pyongyang, which houses the embalmed bodies of Kim Jong-Il and his father and founding president Kim Il-Sung.

In the city's main square, people left floral tributes at the foot of giant statues of the two Kims, with state media saying at least 750,000 mourners had paid their respects on Sunday alone.

The official mourning -- broadcast live on state TV -- saw Kim Jong-Un and his wife Ri Sol-Ju lead rows of officials in bowing deeply to two giant statues of his father and grandfather inside the memorial hall.

The stone-faced officials clad in black were led by goose-stepping soldiers carrying a big floral tribute with a ribbon message reading, "The great comrades Kim Jong-Il and Kim Il-Sung stay with us forever."

A group of scientists who worked on the North's successful long-range rocket launch last week were among the first to pay tribute.

"These are scientists who made great contributions to the successful launch of our Kwangmyongsong-3 satellite and helped showcase the nation's scientific technologies to the whole world," the TV announcer said.

The Kim family has ruled the isolated, impoverished but nuclear-armed nation for more than six decades with an iron fist and a pervasive personality cult.