TAIPEI, May 24 (AP) -- Police detained a Taiwanese Cabinet official Wednesday on suspicion that he illegally profited from an engineering contract linked to a high-speed rail project, prosecutors said.

Police were questioning Hsieh Ching-chih, the vice chairman of the National Science Council, over the awarding of the contract for the project near the southern city of Tainan, an official at Tainan district prosecutors office said.

Hsieh is suspected of having used his influence over the contract in exchange for monetary favors, the official said on condition of anonymity because he was not authorized to comment on the case.

The project aims to reduce vibrations produced by the railway, addressing the concerns of high-tech companies in an industrial zone next to the rail track who fear that tremors from passing trains could damage their delicate products.

The high-speed rail line, which links the capital Taipei with the southern city of Kaohsiung, is scheduled to begin operating by the end of October.

The Cabinet official's detention could hardly have come at a worse time for President Chen Shui-bian; his popularity is plummeting and he already faces a barrage of criticism over scandals involving his family.

Chen's son-in-law, Chao Chien-min, is under investigation for his alleged involvement in insider trading. Also, opposition lawmakers have accused Chen's wife of improperly accepting vouchers from a department store -- a charge the president denies.

Opinion polls show Chen's approval rating at less than 20 percent, the lowest level since he came to power in 2000, partly on promises to clean up government after 50 years of corruption-tainted rule by the once dominant Nationalist Party.