The declaration of the young Kim, believed to be about 27, as the country's top leader came at the end of the 13-day mourning period for his father, who died Dec. 17 after being at the helm of the impoverished, nuclear-armed Communist country for 17 years.
Tens of thousands of mostly soldiers pledged their allegiance to Kim Jong Un after Kim Yong Nam, the country's elderly ceremonial head of state, declared that he was now the "supreme leader of our party, military and people," who inherits the ideology, leadership and courage of his father.
The occasion was preceded by a memorial service to the departed leader.
South Korea's Yonhap news agency said the memorial service was designed to be an official proclamation that Kim Jong Un will lead the communist country.
Kim Jong Gak, a top military officer, announced the country's 1.1 million soldiers "will safeguard comrade Kim Jong Un with their lives," Yonhap reported. The new leader crucially needs the military's support to consolidate his power.
Yonhap said Kim Jong Un, wearing a black coat, made no public comment while watching the memorial service. Military and party leaders were with him.
The New York Times said while questions remain whether Kim Jong Un would be able to take control or depend on caretakers or regents, at least in the public eye he didn't seem he would share his power.
Analysts would also be watching how China, North Korea's main ally and supporter, takes to the new leadership.
Writing in the Wall Street Journal, Chung Min Lee, dean at Seoul's Yonsei University, said from China's perspective, its alliance with North Korea is a burdensome asset, acting as a buffer against South Korea and by extension, the United States and Japan.
"China also recognizes North Korea's usefulness as a political bargaining chip in broader negotiations with the U.S., Japan and others," the article said.
However, the article said those benefits come at a cost as China provides the lion's share of North Korea's food and fuel.
Lee said China's problem is that over time "the costs of bolstering North Korea will grow greater and may start outweighing the benefits.
Lee said China's long term aspirations as a world power may push it to reconsider its policies as a whole toward its three most important Asian allies who are also failed states: North Korea, Pakistan and Myanmar (formerly Burma).