Prominent environmental activist Wu Lihong plunges his hands into a thick layer of toxic green scum and brown foam floating on one of China's biggest freshwater lakes.

Despite a two-decade battle to clean up the once-scenic Taihu Lake that earned him three years in jail, Wu says the water still "stinks" from pollution.

"There's no place in China that isn't polluted. There's no place in China that's clean," complains Wu, 43, who calls himself the "Guardian of Lake Tai".

Taihu Lake lies on the border of the eastern provinces of Zhejiang and Jiangsu in one of China's richest regions, once known as the "land of fish and rice," and is still a source of drinking water for the heavily populated area.

But after three decades of explosive economic growth and lax enforcement of environmental protection laws, the lake -- like most waterways in China -- is heavily contaminated with toxic waste from surrounding factories and farms.

Wu, who remembers swimming in Taihu Lake as a boy, started campaigning to clean up the water in the late 1980s.

The former engineering equipment salesman has paid a high price for his environmental crusade, losing his job in 2001 and landing in jail a few years later as his one-man battle embarrassed and angered local officials.

His once-portly frame has thinned somewhat and his black hair is speckled with white, which Wu attributes to his time in prison.

"There are only a few people like me making a fuss. But I'm not afraid," said Wu, who was jailed in 2007 for extortion.

Wu has long proclaimed his innocence and insisted he was set up by the government of his native Yixing city, which was eager to protect the booming local industry from pollution controls.

Shortly after his arrest, a toxic algae bloom in Taihu Lake contaminated water supplies for more than 2.3 million people in Wuxi city, which sits across the lake from Yixing, drawing nationwide attention to the issue.

Many of Wu's supporters said the toxic pollution, which turned large parts of Taihu Lake a murky green, should have led to the activist's release.

Wu was finally freed last year and was immediately ordered by police to stay quiet or "we will make you disappear".

Authorities, anxious about the power of online social networking sites to organise protests, have also blocked Wu from accessing the Internet.

His rural home is constantly monitored by security cameras, he is routinely followed when he goes out and is not allowed to work.

But Wu says he is undeterred, especially after discovering that the water quality of Taihu Lake improved little during his time in jail despite government efforts to crack down on surrounding factories.

In its 2010 annual report on the country's water quality, China's environmental protection ministry gave Taihu Lake its lowest rating.

Under China's water quality classification system that means the lake is "essentially useless" and can't even be used for agriculture or landscaping, according to a World Bank report.

Wu accuses local officials of protecting factory bosses, who evade detection by discharging pollutants at night, trucking waste to other sites or channelling contaminated water into hastily-dug ditches.

"They are too greedy," Wu said of the officials willing to turn a blind eye to the pollution in the pursuit of economic growth and career advancement.

Local authorities however blame chemicals used by farmers for causing most of the toxins in the water, which have killed fish and other marine life.

The government has made efforts to clean up the lake, releasing millions of algae-eating fish into the water and recently ordering the closure of nearby sewerage discharge facilities and garbage dumps from November 1.

It also pays squads of workers as little as 40 yuan (about $6) a day to patrol the shoreline to collect garbage and suck up algae using a device like a giant vacuum cleaner.

But a layer of green and brown sludge still pollutes the surface of the lake and Wu has vowed to keep up the fight.

"We live here," he said, explaining his dedication. "We could swim in the water before."