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.