Environmentalists view the Three Gorges dam in China, the world's biggest, as a monstrous natural catastrophe waiting to unleash itself on the hundreds of millions of people who live near the Yangtze river.

The Chinese government is fiercely proud of the dam, which is due to open in a few weeks, saying it will stop the river flooding all the time, provide much-needed clean hydroelectric power and give ships from booming coastal cities such as Shanghai better access to central China.

Standing on top of the Three Gorges dam, looking down at the mighty Yangtze flowing below, which the dam seeks to tame, you are more aware than ever before of tension between the desire to maintain ecological balance and the need for progress and energy.

Everything to do with the Three Gorges project, sometimes known as the Great Wall of the Yangtze, is closely monitored - this correspondent was hauled in by the police for talking to a local activist who represents some of the million people whose homes were flooded by the dam and have been relocated to new towns in the region.

The 185-metre high dam goes live in May, months ahead of schedule, and the project is as potent a symbol as you will find of massive social, economic and technological change in China.

It's a stunning creation, and it is astonishing to watch the way the dam manages physically to hold back the third-longest river in the world, or how large container ships are floated up like toy boats by the dam's locks or how the surrounding mountains have been blasted, and towns and countryside flooded, to create the dam.

The dam is 1.4 miles wide, 10,000 people are working on its construction and it will cost pounds 13bn, the government says, while others estimate the real cost is nearer to pounds 40bn.

Beijing proudly trumpets the benefits for the 220 million people who live in the region around the Yangtze and will be served by the huge reservoir it creates.

"There will be an environmental impact but the benefits outweigh the harm and the loss. We had to move one million people and, sure, we flooded some areas but you can't compare the loss of millionsof hectaresof farmland to the safety of 50 million people," said Zhang Shuguang, one of the project's top engineers.

When the dam opens in May, its first function will be flood con- trol - the Yangtze regularly bursts its banks and nearly one million people have drowned in floods in the past hundred years or so. The dam is supposed to stop this happening.

But building the biggest dam in the world involves social change on a massive scale, and even the most fervent backers of the dam agree that relocation has been a tough process.

"The Three Gorgesdamisthe biggest in the world, which means there were issues about technology, investment and migration. I think migration was the most challenging," said Zhang.

The government says one million people have been relocated, while other estimates vary between 1.3 and two million people moved because their homes have been flooded by the rising water of the reservoir.

In one of these new towns, Maoping, the dam forms a dramatic backdrop to a pleasant town where thousands of people were relocated during the 1990s. The new arrivals are pleased with the dam as they believe it will boost China, and they like the new houses, but many have nothingto do in their new homes.

"I used to be a farmer and I lived near a town that is now under water. A lot are doing un-official transportation work, using motorbikes," said one villager. One middle-aged woman, knitting in a group with her friends, said she had a very nice flat which she liked and as whole communities were moved together, she still has her friends around her. "But me and lots of other people have nothing to do."

As we speak we are approached by Fu Xiancai, who says he represents relocated people who have not yet received their full compensation. Predictably, the dam has attracted huge amounts of corruption - one local official was executed in 2000 for taking more nearly pounds 600,000 in bribes and scores of bureaucrats have been arrested for corruption. "Around 80 per cent of the migrant people I talk to are dissatisfied. We've nothing against the project, it's a good dam. But we want our compensation," said Fu.

Later he takes us to see where his house used to stand - it is not under water, but is part of a subsidiary dam near the main project. He has a new house near the site of the old one, complete with Mao portraits.

As two Finnish colleagues, a Chinese editorial assistant and I leave down a dirt road, we are stopped by police and local officials and held for nearly four hours in a chilly government building, before being taken into rooms and interviewed separately. Talking to Fu, no matter how innocently, is a breach of Chinese rules governing how reporters can work. They demand our notes and any photos we have and we are eventually released after signing a statement.

It is hardly surprising that the world's biggest dam should prove a sensitive area in a single-party state. But the dam also sparked the biggest ever political debate in Communist China's history.

The official dispute over whether to build the dam rocked the National People's Congress, China's annual parliament, back in April 1992 and a dam project centre near the Three Gorges has a display showing how nearly one-third voted against the dam or abstained - an unprecedented figure.

Li Peng, then prime minister, who was a fan of the project, declared debate over but final approval was not granted until 1992.

The most famous opponent of the dam in China is the energetic environmental activist and journalist Dai Qing. She opposes the dam because of the lack of public debate about such an enormous project, the fact that the warnings of independent analysts have been ignored and also because she sees it as a huge waste of money.

Her book criticising the project, Yangtze! Yangtze! earned her 10 months in a maximum security prison, during which she was threatened with the death sentence. "Our efforts may look weak and limited in comparison with the government's strong and thunderous media campaign. Whether history proves the project to be a success or a failure, the fact remains that we were simply a group of journalists who took our profession very seriously. We tried to do what we felt was right at a time when we were needed," Dai said.

Environmentalists believe that as the dam slows down the Yangtze, it will lose its ability to generate oxygen, while the waste flowing into the reservoir could turn it into agiant cesspool - 300 mileslong. Silt deposits could also prove a problem as they could choke parts of the river, blocking key ports like Chongqing.

The engineers are upbeat. They point to the fact that hy- dropower is a very clean source of energy compared to the coal- fired power stations that provide the lion's share of China's energy needs. And the government engineers believe the amount of sand and sediment in the river will balance out over the years.

"And of course, cleaning the reservoir is a vital aspect to us, all refuse will be cleaned away and the central government has imposed strict standards on this," said Zhang.

The cultural activists have already lost the battle - most of the 8,000 areas of historical and social interest have all been flooded. A warning here for other areas along the Yangtze perhaps?

Further along the river, construction of Xiloudu dam has begun, which will be the third biggest in the world when it is finished. Three other dams are in the exploration stage near Xiloudu - including one that will flood the beautiful Tiger Leaping Gorge in Sichuan province. All four of these dams together will produce more electricity than the Three Gorges dam.