Prosecutors and legal experts have called for tougher supervision of project investment, especially in rural areas, to prevent opportunities for graft among grassroots officials.

Prosecutors in Beijing's Changping district said they have handled 78 corruption cases since 2008 and eventually convicted 83 of the 98 suspects tried.

"In the cases, 29 defendants were in charge of projects and some were just common employees," said Yang Lin, director of the district's anti-graft department. "In other words, all were far from high-level officials, but had relatively easy access to investment money."

Most cases involved water pipe repairs, road construction and facility building, which involve contract work and offer a high chance for corruption, she said.

Zhang Xuezhu, a former head of a water resources station in Changping, was sentenced this year to nine years in prison for taking more than 80,000 yuan ($13,000) in bribes. He also had embezzled 100,000 yuan, the court heard.

Zhang had been in charge of construction, such as digging wells, and in the previous four years had received thousands of yuan from his illicit interests in construction teams.

"It's not rare to see grassroots officials make use of supervision loopholes to engage in corrupt practices and ask for large sums of money from bidding companies," Yang said.

Zhang Tao, a prosecutor in the capital's Shijingshan district, said the authority is cooperating with several large companies to supervise their management and find signs of corruption or bribery.

"Sometimes a few big enterprises prefer to hide their problems rather than face them, since they think it will affect their reputation if the case is disclosed," he said.

"We provide regular training and legal knowledge for the supervisors in enterprises, letting them know the report will be good to their workplace and make a contribution to the group's development."

Similar cases also occurred in Beijing's Fangshan district, where prosecutors have tried 15 local officials in the past five years leading to criminal punishment for 11 of them.

Wang Jianming, director of the district's anti-graft division, said many cases of bribery and corruption involved village heads.

Ma Guidong, a former village head in the district, was sentenced to 11 years in prison for corruption after he committed identity fraud on four occasions to get compensation for the South-to-North Water Diversion Project. Ma received more than 520,000 yuan between 2006 and 2008.

Channels for villagers to supervise a head's financial affairs are limited, and some grassroots officials only disclose the total investment of a project instead of a detailed breakdown, Wang said. He added: "It's a must to supervise projects, strictly control bids and verify accounts in time."

Yi Shenghua, a Beijing lawyer with 10 years' experience in corruption cases, said the government should be transparent when it comes to finance, project investment and officials' assets.

"Grassroots officials directly use public money and contact bidders. That's why they can take bribes and be corrupt easily," he said.