BEIJING — A human rights advocate from Taiwan has been detained in China on suspicion of being a threat to national security, an official in Beijing said on Wednesday, adding to signs of an intensified clampdown on outsiders working with China’s beleaguered human rights lawyers and groups.

The activist, Lee Ming-cheh, went missing more than a week ago, after he took a flight from Taipei to Macau to cross into mainland China. His case had already ignited widespread news coverage in Taiwan, a self-governed island at odds with mainland China, which claims the island as its own territory. That unease appeared likely to grow after a Chinese official confirmed that Mr. Lee was being held on a highly serious charge.

Mr. Lee was “under investigation by the relevant authorities on suspicion of activities harmful to national security,” Ma Xiaoguang, a spokesman for the Chinese government’s Taiwan Affairs Office, said at a news conference in Beijing.

Mr. Ma did not detail what activities brought Mr. Lee under suspicion. But they probably involved his contacts in China, where he used social media to discuss Taiwan’s transition to democracy, visited friends about once a year, and delivered donations of books and money to the families of imprisoned human rights lawyers, according to his wife and to the director of the community college in Taiwan where he is employed.