A Chinese lawyer involved in Ai Weiwei's tax case said Wednesday police were probing accounts at his law firm, just as the controversial artist and activist's wife was taken in for questioning.

Pu Zhiqiang, the lawyer for a company founded by Ai that was accused of huge tax evasion after the artist was released from 81 days in detention earlier this year, said police had visited his Huayi Law Firm on Tuesday.

"They said they wanted us to assist them in solving a case, and photocopied this year's account documents," Pu told AFP, adding he did not know if the visit was related to Ai Weiwei.

"There is no evidence that they are targeting me," he said, but refused to discuss the specifics of the case.

The police visit happened on the same day that Ai's wife Lu Qing was summoned by police and interrogated for three hours as a "criminal suspect", although it is unclear what crime she is suspected of.

Ai, whose activism has made him a thorn in the side of China's Communist authorities, was charged on his release in June with tax evasion linked to Fake Cultural Development, a firm he founded but that is legally owned by his wife.

Pu, one of Fake's lawyers, has helped Ai fight the charges, which the artist denies, calling them "politically motivated".

Earlier this month, the 54-year-old was handed a bill for 15 million yuan ($2.4 million) in alleged back taxes.

Within weeks of receiving the bill, he was able to pay an 8.45 million yuan guarantee needed to challenge the charge thanks to donations from fans and supporters of his activism.

During her interrogation, Lu said police had asked her about Fake and told her not to leave Beijing.

Beijing's police refused immediate comment on both Ai's case and the investigation into Pu's law firm when contacted by AFP.

The artist's problems with authorities started when he began investigating the collapse of schools in the 2008 Sichuan earthquake and a 2010 fire at a Shanghai high-rise that killed dozens.

But until his detention he had been left relatively unscathed thanks to his family background. His father is the late Ai Qing, a famous poet who was disgraced and later rehabilitated by the Communist regime.

This week, the influential US-based periodical Foreign Policy ranked Ai number 18 on the magazine's list of top 100 global thinkers.