Revered by the Vietnamese, the creature has been made ill by pollution, but Hanoians are now struggling to clean its lake

Hundreds of people are working round the clock to clean up a lake in the heart of Vietnam's capital in hopes of saving a rare, ailing giant turtle that is considered sacred.

Experts say pollution in Hanoi's Hoan Kiem lake is killing the giant freshwater turtle, which has a soft shell the size of a desk. It is one of the world's most endangered species: it is believed that there are only four in the world.

Teams of people are cleaning debris, pumping fresh water into the lake and building an artificial island to serve as a "turtle hospital". The rescuers may try to net it for the first time as part of the effort.

The Hoan Kiem turtle is rooted in Vietnamese folklore, and some believe the creature that lives in the lake today is the same mythical turtle that helped a Vietnamese king fend off the Chinese nearly six centuries ago.

The turtle in Hanoi's lake swims alone and in the past has been glimpsed only rarely, sticking its neck out of the water, but it has recently surfaced much more frequently, alarming the public with glimpses of raw open wounds on its head and legs.

Meetings were called, a council was established and 10 government agencies were put to work to try to save it.

"For the Vietnamese, the Hoan Kiem lake turtle is the most sacred thing," said retired state employee Nguyen Thi Xuan, 63, who had travelled from a suburban district to try to get a glimpse. "He has helped the Vietnamese to defeat foreign invaders and helped the country to have peace. I hope he will live for ever."

The lake, which is one mile (1.6km) long, is a city landmark, with its curved red bridge leading to a temple on a tiny island. Weeping willows and other leafy trees shade the pavement on the road that circles the water. It is a popular site for tourists and Hanoians to exercise and relax.

But all sorts of rubbish have been thrown into it, including bricks, concrete, plastic bags and raw sewage. It is not uncommon to see men urinating directly into the murky water.

The pollution is slowly killing the Hoan Kiem turtle, a Vietnamese scientist said.

"I believe the injuries were caused by sharp edges from debris in the lake," said associate professor Ha Dinh Duc, who has studied the lone turtle for 20 years and considers himself its caretaker. "The poor quality of the water also makes the conditions unbearable for the turtle."

Duc said small red-eared turtles, which are popular pets, have also been released into the lake. They are believed to be feeding on the giant turtle's festering wounds, which may be worsening the infection.

The turtle rescue team hopes to coax it on to land and treat the wounds.

Sandbags have been built up to create a small island for it to emerge. But if it does not crawl on to the platform by itself, a net will be used to capture it.

No one knows the turtle's age or sex, but scientists say it is probably the most endangered freshwater turtle species in the world. It weighs about 200kg and its shell is 1.8m long and 1.2m wide.