The world's biggest dam is to open in May, months ahead of schedule. The Three Gorges dam is viewed by supporters with pride as a symbol of China's economic and social change but environmentalists believe it is a catastrophe waiting to happen.

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 £13bn, the government says, while others estimate the real cost is nearer to £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 millions of hectares of farmland to the safety of 50 million people," said Zhang Shuguang, one of the project's top engineers.