Hanoi (AFP) - A sacred giant turtle venerated as a symbol of Vietnam's independence struggle has died, state media said, prompting an outpouring of grief and fears the death bodes ill for an upcoming communist leadership handover.

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

Experts say it was one of only four turtles -- better known as Yangtze giant softshells -- in existence. Two are in a zoo China and the other lives in a different lake in Hanoi.

It was found dead in Hoan Kiem lake in central Hanoi late Tuesday, the state-run Tuoi Tre newspaper said.

The turtle, which weighed about 200 kilograms (440 pounds), was said to be anywhere between 80 and more than 100 years old.

It was "possibly the rarest species on the planet and definitely the rarest turtle species... It's a great blow," to have lost it, Tim McCormack of the Asian Turtle Program, told AFP.

"It was clearly an ancient animal, I would say easily over 100 years old, and so its death does not come as any real great surprise," he said.

Because of the cultural significance of the turtle, which many Vietnamese considered "almost sacred", it has not been possible to include the Hoan Kiem animal in any breeding programmes which could have helped conserve the species, he added.

The turtle's demise was widely mourned on Vietnamese-language blogs and social media, with many warning it was a bad omen for upcoming changes in the ruling Communist Party, which begins its five-yearly congress on Thursday.

"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 party congress.

In a story taught to all Vietnamese schoolchildren, 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.

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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 Tuesday, but some were taken down 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.