Adam Brookes BBC Beijing correspondent

Much criticism has come from inside China

The $25bn project is the largest hydro-electric power project in the world, but years of controversy have surrounded its building.

The Three Gorges dam looms 300 metres above the Yangtze River in central China.

Its construction won't be completed until the end of the decade, but on Sunday the water level in its reservoir will be allowed to start rising and in the coming years the dam will create a lake 600 kilometres long.

More than 600,000 people have already been resettled from areas that will be flooded. Entire towns will disappear.

China's Communist Party leaders have portrayed the dam as a triumph of engineering and of socialism.

Hundreds of thousands have had to find new homes

They say it will mean an end to catastrophic flooding along the Yangtze and it will provide hydroelectric power to a thirsty Chinese economy.

But the dam's detractors are many and vocal. Environmentalists warn of serious damage to the eco-system; engineers have expressed worries about construction standards, and the project has been dogged by allegations of corruption.

Unusually, much of this criticism has come from inside China; the project has been a lightning rod for dissent here.