A giant turtle's death has triggered a sense of foreboding among Vietnamese even as a secretive meeting of the country's communist party was about to start to select new leadership.

The 'grandfather' turtle, which is thought to be only one of four living creatures from a nearly extinct turtle species, was found dead in Hoan Kiem lake in central Hanoi late on Tuesday.

The death of the Swinhoe's softshell turtle, which weighs close to 200 kg and is aged approximately 100 years, caused an outpouring of grief with people linking the death to likely untoward happenings.

"This is bad news for many people in Hanoi," the Thanh Nien newspaper said, according to ABC.

While another giant turtle from the species better known as Yangtze giant softshells lives in a different Hanoi lake, two others are in a zoo in China.

According to school textbooks in Vietnam, the sacred turtle of Hoan Kiem, which died on Tuesday, is the custodian of the magic sword of Le Loi, a 15th-century rebel leader, AFP reported.

Many Vietnamese who shared grief on social media said they feared the death of the turtle was a bad omen for the party congress. For some people the development signalled a Chinese invasion while others thought it hinted at the collapse of the Communist party.

The communist party delegates will choose a new party general secretary as well as president and prime minister at the party summit. They will also finalise the social-economic plan for the next five years.

The turtle's body is kept at a temple as authorities finalise plans for a ceremonial burial.