BEIJING—China accused the artist-activist Ai Weiwei of evading "a huge amount" of tax through a company he controlled, in the most detailed explanation yet of the charges against the prominent government critic who was detained six weeks ago.

Police investigators also found that Beijing Fake Cultural Development Ltd.—the company that handles Mr. Ai's financial affairs—had intentionally destroyed accounting documents, according to a report published by the state-run Xinhua news agency late Friday.

Mr. Ai's relatives didn't respond to telephone calls, but they have previously dismissed similar accusations, and his wife, Lu Qing, says the company in question is registered and belongs to her, according to the Associated Press.

The Xinhua report gave no further details of the charges against Mr. Ai, who was detained at Beijing's airport on April 3 shortly before he was due to board a flight to Hong Kong and wasn't allowed to communicate with his family until this past Sunday, when he met his wife.

But Xinhua added that Mr. Ai's right to meet with "people who live with him" was guaranteed under Chinese law while he was under "residential surveillance"—a form of house arrest that is often used in politically sensitive cases in China.