Chinese censors are apparently blocking online discussion of former president Jiang Zemin, whose absence from a key Communist Party event in Beijing has sparked speculation that he is seriously ill.

The 84-year-old Jiang was conspicuously missing Friday from state celebrations marking the 90th anniversary of the founding of China's Communist Party. The Hong Kong economic journal Hsin Pao quotes sources as saying Mr. Jiang is thought to be undergoing treatment at Beijing's 301 Military Hospital.

China's Twitter-like Weibo platform on Wednesday blocked terms such as “Jiang Zemin,” “myocardial infarction” and “general secretary,” sparking a wave of online speculation about the former president's well-being.

Wednesday, searches for the term “Yangtze River” also come up blank, as did searches for a number of other Chinese rivers. Mr. Jiang's surname means “river,” and Weibo posted an official notice saying the results of such searches can not be displayed.

China's Foreign Ministry refused to comment when asked Wednesday about Mr. Jiang's status.

Beijing has long guarded information on the health of the nation's top leaders as a state secret.

Mr. Jiang is seen as one of China's most prominent and influential political figures since paramount leader Deng Xiaoping, who died in 1997. Mr. Jiang oversaw the peaceful return of Hong Kong from British rule in July of that year, and is widely credited with a spate of economic reforms and strategies that helped transform the country into a global economic power.