BEIJING — North Koreans had long known Jang Song-thaek as the No. 2 figure in their country, the revered uncle and mentor of Kim Jong-un, the paramount leader. Then on Monday state-run television showed two green-uniformed guards clutching a glum-looking Mr. Jang by the armpits and pulling him from a meeting of the ruling party after he was denounced for faction-building, womanizing, gambling and other acts as dozens of former comrades watched.

The spectacle of Mr. Jang’s humiliating dismissal and arrest was a highly unusual glimpse of a power struggle unfolding inside the nuclear-armed country. But the major impact may be outside, and nowhere is the downfall more unnerving than in China.

North Korea’s longtime protector and economic lifeline, China has considered strategically close relations with North Korea a pillar of foreign policy and a bulwark against the United States military presence in South Korea. Despite Chinese irritation with North Korea’s nuclear tests and other bellicose behavior, China had built a good relationship with Mr. Jang as the trusted adult who would monitor Mr. Kim, who is less than half his age.

Any shift by China concerning North Korea has the potential to significantly alter the political equilibrium in Asia, where the divided Korean Peninsula has been a fact of life for more than 60 years. While there is no indication that the Chinese intend to change their view, it seemed clear that even Beijing’s top leaders were surprised by Mr. Jang’s abrupt downfall on Sunday, and even more on Monday by the North Korean state television broadcast.