China has completed construction of the main wall of the Three Gorges Dam - the world's largest hydro-electric project.
The controversial dam in central Hubei province will not be fully operational until 2009, once all its generators are installed.
China says it will provide electricity for its booming economy and help control flooding on the Yangtze River.
Critics say over a million people were moved from the area, and the reservoir behind the dam is already polluted.
On Saturday, builders poured the last amount of concrete to complete the construction of the 185m (607ft) high, 2,309m (1.4 mile) long wall.
A senior Chinese official said the event marked a "landmark progress" in the dam's construction, the state-run Xinhua news agency reported.
"However, tasks such as building of power houses of the dam, the ship lock and shiplift are still formidable," said Pu Haiqing, deputy director of the dam's construction committee.
When its 26 turbines become operational in 2009, the dam will have a capacity of more than 18,000 megawatts.
Already the world's second-largest consumer of oil, China says it needs alternative energy sources to combat widespread power shortages and keep its booming economy powering along.
LARGEST HYDRO-ELECTRIC DAMS
The authorities also hope the dam will help control flooding on the Yangtze River, which in the past has claimed hundreds of thousands of lives, the BBC's Quentin Somerville in Shanghai reports.
But campaigners say the dam comes at too high a cost.
Over a million people have been moved from their homes to make way for the project and more than 1,200 towns and villages will disappear under its rising waters.
Environmentalists say the water behind the dam is already heavily polluted.
China says the whole project will cost about $25bn (£13bn), but environmentalists estimate it to be several times higher.
THE THREE GORGES DAM
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