BEIJING — Jang Song Taek, the uncle by marriage of North Korean leader Kim Jong Un, was executed for attempting to seize power for himself, the official news service reported early Friday.

In a rambling report, the Korea Central News Agency denounced the "despicable human scum Jang, who was worse than a dog," and said he had attempted to "overthrow the state by all sorts of intrigues and despicable methods with a wild ambition to grab the supreme power."

PHOTOS: Kim Jong Un uncle labeled traitor

The execution, after a trial this week by a military tribunal, is the most public purge of a top echelon official in North Korea in decades and raises concern about the increasingly erratic leadership of 30-year-old Kim Jong Un, believed to be the world's youngest head of state.

"Such a quick trial and execution also seem to show the instability of the regime. There will be many more executions of Jang's allies and confidants," said Park Syung-je, a North Korea expert and chairman of South Korea's Asia Strategy Institute.

The 67-year-old Jang was married to Kim Kyong Hui, the daughter of founder Kim Il Sung and the only full sibling of the late leader Kim Jong Il. He was widely considered the second-most important person in the country and had installed numerous relatives and cronies in key positions in the government and military.

Since the Kims were famously insecure about leaving North Korea, Jang frequently represented the country abroad, even visiting Seoul in 2002, where he impressed South Korean officials with his economic savvy. He also was a key interlocutor with the Chinese, who considered him the most reliable figure in an eccentric leadership.

North Korean defectors in Seoul say Jang's wife, who is suffering from acute liver disease, assisted in bringing down her husband to maintain the hereditary "blood line" of the Kims.

A report by the North Korea Strategic Information Service Center, run by a defector, Lee Yun-keol, said Kim Jong Un's brother Kim Jong Chul, 32, held the gun when two of Jang's underlings were arrested.

Whether the report of the execution is true, it indicates that Kim Jong Un aims to explain the purge to his advantage and avoid political repercussions from Jang's supporters.

"Normally when the North Korean leadership feels threatened by a person, they will get rid of him secretly and say he was diseased of cancer or accident. However, the regime made Jang's case very public, which shows Kim Jong Un's determination that anyone who gets in the way, even family, will be punished severely," Park said.

On Sunday, Kim presided over a special meeting of the politburo at which Jang was stripped of his posts and, in a spectacular televised scene, yanked by his armpits and dragged out of a chair.

Although the late Kim Jong Il appeared jealous of Jang's power and influence, before his death in December 2011 he elevated him as a mentor to groom Kim Jong Un.

"It is quite shocking, but it seems like Kim Jong Un was determined to show [that] whoever challenges his sole leadership will be punished, without mercy," Kim Yong-hyun, a North Korea studies professor at Dongguk University in Seoul, told South Korean news channel YTN.

The professor added that the timing of the execution, just before the Dec. 17 anniversary of Kim Jong Il's death, will serve as confirmation that the era of the younger generation has begun.

At least two other officials — perhaps dozens — had already been executed, in what South Korean media are referring to as the "bloody purge."

Many diplomats, including the ambassadors to Cuba and Malaysia, both relatives of Jang, have been recalled. The ambassador to Beijing, a Jang protege, is also expected to be recalled, along with many of the officers in the army-run companies that handled trade with China.

Chinese experts say the upheaval is particularly alarming because North Korea has nuclear weapons and a fragile economy.

Among other charges, Jang was accused of selling national resources at low prices, presumably to the Chinese. He was also accused of "womanizing and debauchery," allegations that might have played into his wife's reported decision to allow his execution.

Jang was convicted under Article 60 of the criminal code, which bars acts against national sovereignty. The official news agency's report accused him of factionalism, which in communist states means deviating from party unity.

Jang's death sentence was declared Thursday by a special military tribunal of the Ministry of State Security, the report said.

U.S. officials said they could not confirm the execution but had no reason to doubt the reports.

"If confirmed, this is another example of the extreme brutality of the North Korean regime," State Department spokeswoman Marie Harf said. "We are following developments in North Korea closely and consulting with our allies and partners in the region."

Despite the turmoil, former NBA star Dennis Rodman, who has built a friendship with Kim Jong Un, a basketball fan, is set to visit the nation next week. Rodman is organizing a friendship exhibition game between North Korea and a team of mainly former NBA players on Jan. 8, Kim's birthday. Sponsor Paddy Power, an Irish bookmaker, told Agence France-Presse the plan was not affected by the recent political events in North Korea.

barbara.demick@latimes.com

Times staff writer Demick reported from Beijing and special correspondent Choi from Seoul. Times staff writer Paul Richter in Washington contributed to this report.