The reopening ceremony for Kumsusan Palace of the Sun, held from 9:00 a.m., was broadcast live by key television stations including (North) Korean Central Broadcasting Station, monitored in Seoul.
The North's state news wire service Korean Central News Agency said in an English dispatch that "an inaugural ceremony of the renovated Kumsusan Palace of the Sun is now under way in a solemn atmosphere on Monday."
Television images showed North Korean leader Kim Jong-un attending the ceremony held at the mausoleum, where his father's embalmed body is laid, along with top military, party and government figures.
First lady Ri Sol-ju was with the leader when he paid tribute there. Clad in a black mourning dress, Ri appeared to be in the late stages of pregnancy as South Korean media have widely speculated, according to the television images.
Scientists and others who contributed to the North's successful launch of a long-range rocket on Dec. 12, foreign diplomats, soldiers, laborers, farmers as well as party, military and government officials were invited to the ceremony, the television station said.
Earlier this year, the North embarked on the renovation work at the Kim family mausoleum where the body of the country's founder Kim Il-sung is also laid.
The re-opening ceremony is widely expected to be followed by the unveiling of the preserved body of the late leader for public viewing.
The North said Kim Jong-il died of a heart attack on Dec. 17 while traveling for a field guidance. Kim's third-son Jong-un took over power following the death.
KCNA also released the North Korean leader's thank-you letter to mausoleum renovation workers in which Kim urged soldiers and other North Koreans to "transform our country into an economic power" in order to fulfill his father and grandfather's hopes of establishing a prosperous socialist nation.
The news agency said the country sounded a siren across the country and had three minutes of silence as part of commemoration gestures for the late leader.
The Monday issue of the country's Rodong Sinmun had two additional pages filled with commemoration content.
Monday's ceremony also drew wild media speculations as a mystery man was next to North Korean leader Kim Jong-un at the official state event.
The middle-aged man stood right next to Kim when the group of top officials paid tribute together to the statues of Kim Jong-il and North Korean founder Kim Il-sung in the mausoleum. The position used to be taken by Choe Ryong-hae, the director of the People's Army General Political Bureau.
The mystery man was also shown standing next to Kim at Sunday's commemoration event for the late leader.
North Korean figures' positions at official events are key signs indicating their ranks and levels of importance in the regime.
Experts said the man could be a key figure related to the Dec. 12 launch of what the country says was a rocket carrying a "working satellite" or security service for the leader.
"The man is a new face who hasn't been seen before. We are confirming his identity," a Seoul government official said.