Reports of the death of the former president Jiang Zemin have been greatly exaggerated, the Chinese state media has insisted, amid a frenzy of speculation online and overseas.

In an unusual move, state news agency Xinhua issued a brief denial that the 84-year-old statesman had passed away to quell rumours that began on Friday when Jiang failed to attend the biggest political event of the year – a 90th anniversary celebration to mark the founding of the Chinese Communist party.

Asia Television of Hong Kong broadcast a report on Wednesday claiming Jiang had died of an unnamed illness. Japanese and South Korean media issued similar bulletins. Chinese journalists said they had been told to expect news on Thursday, but the only comment was a single line from Xinhua.

"Recent reports of some overseas media organisations about Jiang Zemin's death from illness are pure rumour," the newswire said, quoting unnamed "authoritative sources".

Despite the denial, speculation is unlikely to fully die down until Jiang is seen again in public. The last such big occasion was in October 2009 for the 60th anniversary of the founding of the People's Republic. Jiang was also present at an event in Sichuan last year, where his car was reportedly followed by an ambulance. Jiang is said to be suffering from an illness and possibly hospitalised, although it is hard to confirm because China treats the health of its leaders as a state secret.

Until today, censors have tried to quell speculation by blocking references to related words on blogs and search engines: "Jiang", "myocardial infarction", "hung" – a euphemism for death – and "301 Hospital" – a reference to the medical facility where he was said to be treated.

Searches for "jiang", which means river, resulted in the warning: "Search results are not shown due to relevant laws and policies." Netizens sidestepped the prohibition by using English words and allusions.

The US-based dissident news site Boxun.com said Shandong News in eastern China had its website disabled by authorities for reporting Jiang's death, though the newspaper dismissed Boxun's report.

Boxun showed what it said was a screenshot of Shandong News with a banner headline reading "Venerable Comrade Jiang Zemin Will Never Be Forgotten" next to a photo of the former leader.

A woman in the news department at Shandong News said the newspaper's site went offline on Wednesday because its servers crashed and it was still trying to fix the problem.

The woman, who would only give her surname, Wang, said the website never posted news saying Jiang had died.

"That's a rumour," she said. "Maybe someone with ulterior motives made that screenshot."

The internet cat-and-mouse game over the possible death of a former leader underscores how secretive China's Communist party leadership remains – and the difficulties of maintaining that secrecy in a well-wired society.

Jiang Zemin rose to power in the wake of the 1989 Tiananmen Square killings, oversaw the handover of Hong Kong in 1997 and led his country until 2002, by which time it was one of the world's most powerful economies. He was replaced by the current president Hu Jintao.