A sacred giant turtle venerated as a symbol of Vietnam’s independence struggle has died, state media have said, prompting an outpouring of grief and fears the death bodes ill for a forthcoming communist leadership handover.

The reptile, a critically endangered swinhoe softshell turtle, occupies a key mythological role in the country – in the past, the turtle surfaced only rarely, with its sightings deemed auspicious.

Some scientists believe it was one of only four of the species – better known as Yangtze giant softshells – in existence. Two are in China; the other lives in a different lake in Hanoi.

The reptile was found dead in Hoan Kiem lake in central Hanoi on Tuesday, the state-run Tuoi Tre newspaper said. The turtle, which weighed about 200kg (440lbs), was said to be 80-100 years old.

Its demise was widely mourned on Vietnamese-language blogs and social media, with many warning it was a bad omen for forthcoming changes in the ruling Communist party, which begins its five-yearly congress on Thursday.

The giant turtle surfaces in February 2011. The rare sightings of the reptile were deemed auspicious. Photograph: Luong Thai Linh/EPA

“This is bad news for many people in Hanoi,” said the Thanh Nien newspaper. Vietnam’s authoritarian rulers will choose a new party leader, president and prime minister at the congress.

In a story taught to all Vietnamese school children, the sacred turtle of Hoan Kiem is the custodian of the magic sword of Le Loi, a 15th-century rebel leader who vanquished Chinese invaders.

Although officially an atheist country, many Vietnamese are deeply superstitious. “I feel empty. My children, grandchildren will only know the turtle from legend,” online commentator Duong Nguyen wrote on the popular VNExpress site.

Reports about the turtle’s death first appeared in state media late on Tuesday, but some were removed, apparently under pressure from communist authorities.

The turtle’s body is being kept at a temple on a small island in the lake pending an official decision on how to proceed, state media said, adding that embalming was being considered.