Malaysia says it arrested a suspected senior figure in a network that traded in nuclear secrets because his presence in Malaysia exposed the country to possible attacks and economic sanctions. Malaysian authorities arrested Buhary Syed Abu Tahir, a Sri Lankan national with Malaysian residency, under the country's Internal Security Act, which allows for indefinite detention without trial. Deputy Internal Security Minister Noh Omar said Saturday that Mr. Tahir was detained because he was involved in illegal activities at an international level, as he put it, "by involving himself in an illicit international network of nuclear proliferation."

Malaysia was upset last February, when President Bush, in a speech, mentioned Malaysia and Mr. Tahir's activities there several times. The Malaysian government questioned Mr. Tahir at that time, but declined to arrest him.

Mr. Noh denied Saturday that the arrest was made under pressure from Washington. He said the government felt Mr. Tahir's continued freedom left the country open to "threats of attacks from big powers and economic sanctions." He did not say which "big powers" he was referring to.

Washington reacted positively. Richard Boucher, the U.S. State Department spokesman, said the United States was "delighted" at the Malaysian action. "Mr. Tahir was one of the most important figures in the AQ Khan network," he said. "He served as chief financial officer and, essentially, ran network operations. We think his arrest is a major step, and it will serve as a catalyst to international efforts to shut down the Khan network."

Mr. Tahir is alleged to be a central player in the illegal nuclear network established by the former head of Pakistan's nuclear weapons program, Abdul Qadeer Khan. Mr. Khan, a onetime national hero who is credited with making Pakistan a nuclear power, admitted earlier this year to selling nuclear secrets to Iran, North Korea, and Libya. He was pardoned after apologizing publicly to the country for his wrongdoing.

Mr. Noh said Mr. Tahir had used Malaysia as "an illegal base" to manufacture centrifuge parts for Libya's now-abandoned nuclear weapons programs. He also said Mr. Tahir had clandestinely brought technicians from Libya to Malaysia to receive training.

The company responsible for making the centrifuge parts is controlled by the son of Malaysian Prime Minister Abdullah Ahmad Badawi. At the time Mr. Tahir was first publicly identified, the prime minister said Mr. Tahir had committed no crime in Malaysia, and the government, therefore, refused to arrest him.

Mr. Noh, the deputy internal security minister, said there was no pressure from any other country since then to make the arrest. He said Malaysia would consider any request by Washington to interview Mr. Tahir.