After decades of engineering studies and political challenges, China has started construction on the Three Gorges Dam, the largest hydroelectric project in the world, which is to tame the storied floodwaters of the Yangtze River valley.

Although excavation began this month, project engineers acknowledge that uncertainties still abound over the mammoth undertaking, which will become the largest single drain on China's capital budget for more than a decade. The dam is scheduled to be completed in 2009.Engineers say that significant problems remain on how to finance the $17 billion cost of the dam, how to relocate more than 1 million Chinese and how to lessen the effects of sewage pollution and heavy sedimentation on large upstream cities like Chongqing, whose metropolitan area has a population of 15 million.

In a ceremony that seemed hastily organized and to which foreign news organizations were not invited, Premier Li Peng flew to the dam site in the city of Yichang in central China this month to proclaim that construction had begun. But other top leaders were conspicuously absent.

President Jiang Zemin, who is also general secretary of the Communist Party, did not attend. In October, during a tour of Sichuan Province, Jiang appeared to have reservations.

"Some scientific appraisals of the project still need to be intensified continuously," he told local party leaders. He acknowledged that the large-scale resettlement effort would be "arduous" and admonished project engineers that "we should insure" all aspects of the design "will be perfectly safe."

Li, an electrical engineer who serves as head of the Cabinet and as chief promoter of the project, may have chosen this juncture to announce a construction start because of the failing health of China's paramount leader, Deng Xiaoping. Many Chinese believe that Li and his policies will be more vulnerable to challenge after Deng's death.

At a news conference this month, Guo Shuyan, a project official, said China faces a shortfall of at least $3 billion in financing the dam.

The government, he said, is confident it can raise the money by issuing international bonds, but the market for those bonds has not been tested.

"Construction of the Three Gorges Dam is not only of strategic importance to China's development," Guo said, "it is also an opportunity for the international financial, industrial and business sector."

The message of the news conference was a call to large Japanese, American and European firms in the power-generation business to join the bidding to provide the Three Gorges project with the huge turbines, transformers and other heavy machinery it will require. And, Chinese officials made clear, all bidders will have to be willing to share their technology and bring their own financing for the project.

In statements that highlighted the challenges that remain, Premier Li and Guo both said that China does not have the technology to build 26 sets of 700-megawatt turbines and electrical generators that will be the heart of the dam and its putative capacity to generate 18,200 megawatts of electricity, making it significantly larger than the Grand Coulee Dam in the United States, whose rated output is 10,830 megawatts.

"Manufacturers and businessmen of large hydropower and transformer equipment from around the world are welcome to take part in the construction in the form of technology transfer or cooperation," the premier told an audience of party cadres in Yichang.

This fall, more than 14,550 people were uprooted from homes in the immediate area of the construction site. An earthen berm to block the flow of the river will be completed by 1997 and the first group of generating units is scheduled to begin producing electricity by 2003.