Experts in Hanoi have captured a legendary giant turtle for medical treatment, a milestone in a case that has grabbed national attention and cast a spotlight on environmental degradation in Vietnam.

Several dozen people, including special forces soldiers, swam and used boats to pull three rings of nets around the rare beast - the size of a car door with a head as big as a human's, from a lake in the heart of the Vietnamese capital.

In recent months the turtle has surfaced almost twice as frequently as in previous years, according to state-run media, and what appear to be pink sores have been photographed on its shell, neck and feet, raising alarm.

Experts believe the turtle, weighing some 200 kilograms, may be suffering the effects of pollution, snaggings by fish hooks, attacks from smaller red ear turtles that have proliferated in the lake, or all three.

"Generally, the turtle is fine and stable," turtle expert Ha Dinh Duc, part of the capture operation and a member of a steering committee set up to make decisions about the elderly animal, said after it was ensnared.

Thousands of people crowded around the lake to watch.

According to Vietnamese legend, a giant turtle in the same lake - some believe even the same animal - reclaimed a magic sword given to King Le Loi in the 15th century that he used to win independence from China's Ming Dynasty.

The lake was thereafter named Ho Hoan Kiem, or Lake of the Returned Sword, and Hanoi has been built up around it.

Scientists have said they believe the turtle caught on Sunday may be more than 100 years old and a member of the critically endangered Rafetus swinhoei family of turtles, although others have dubbed it an independent sub species.

The case has made headlines in the local media for months, sparking some criticism over the state of the environment in fast-developing Vietnam.

"We have been too apathetic with regard to the turtle in the past as we rolled along with the speed of industrialisation and modernisation and forgot to protect its habitat," resident Le Tuan posted on the popular news website Vnexpress.vn.

Critics also say it took scientists and the government too long to decide how to care for the animal.

The reptile tore a hole in a net on March 8 and escaped when workers waded into the algae-green waters of the lake in a first attempt to catch it.

On Sunday the turtle appeared to get past two of the nets, but a shirtless swimmer believed to be from special forces grabbed the shell and rode the animal a short while to prevent it from getting past the third net, television footage showed.

It was put into a pen on an island in the middle of the lake that has been dubbed its hospital and sanatorium.

-Reuters