BEIJING, JUNE 27 -- China's president, Yang Shangkun, is recovering from an operation for acute appendicitis, it was revealed today in a reminder of the frailty of Beijing's octogenarian rulers and the uncertainty of the country's impending leadership succession.

Vice President Wang Zhen, 82, during a meeting today with Chad's president, Hissene Habre, disclosed that President Yang, 83, had the operation Monday.

Premier Li Peng told Habre that he visited Yang in the hospital Tuesday and found him to be very well and recovering, the official New China News Agency reported.

Some Chinese call Yang the second most important leader in the country after supreme leader Deng Xiaoping, 85, who has ostensibly retired.

After Zhao Ziyang was dismissed from his post as party general secretary last June because of his support for the student-led democracy movement, Deng formally designated Jiang Zemin as his successor. Deng referred to Jiang, who became the new party chief, as the party's new "core leader" and instructed both hard-liners and reformers to unite behind Jiang.

But many Chinese regard Yang as more powerful than Jiang because of the president's connections within the military. Jiang, 63, was named last year to head the party's Central Military Commission. But Jiang had no military experience, and has no base of support within the armed forces.

But Yang is a veteran of the Red Army's legendary Long March across China in 1934-35 and has held a series of military posts. He is first vice chairman of the military commission, the number-two position in the armed forces.

His brother, Yang Baibing, 70, is number four in the military as secretary general of the commission. As head of the army's General Political Department, which oversees personnel matters, Yang Baibing is believed to have been involved in the recent reshuffle of six of seven regional military commanders.

According to military analysts, the shake-up was aimed in part at preventing regional commanders from building power bases. Among those rewarded with key posts were officers who played leading roles in the brutal army crackdown on the democracy movement. Both Yangs are believed to have played important roles in suppressing the movement.

Chinese with contacts in the military say some younger military officers resent the Yang brothers. Critics inside and outside the military have accused the brothers of nepotism. During the democracy movement, university students alleged that the two were attempting to take power by creating a "Yang family army."

Yang Shangkun is both a comrade in arms and potential rival of Deng, the country's top leader for more than a decade. Deng formally retired as head of the military commission last year but is still regarded as the final arbiter in settling important matters of state.

The Yang brothers have been highly visible in recent months. President Yang, appearing vigorous despite his age, made a five-nation trip to Latin America last month, the first trip to Latin America by a Chinese head of state.

As the army's top political commissar, Yang Baibing is often seen rewarding model soldiers and lecturing officers on the need for loyalty to the Communist Party.