Chinese police are investigating Ai Weiwei on pornography charges, the artist said Friday, in the latest move against the outspoken government critic following his detention and a massive tax bill.

The latest accusations centre on old pictures posted online of the activist -- who spent 81 days in secret police detention earlier this year and was later accused of evading taxes on a huge scale -- posing with naked women.

"Yesterday they took my assistant to the police station. They (police) clearly told him this is an investigation, now, they are doing on me, on pornography," Ai told AFP by telephone.

Ai said authorities had accused him before of producing pornography, but he had not taken the charge seriously.

"When they detained me, they said 'this is pornography', but I just laughed, I said, 'do you know what is pornography'?" he said. "Nudity is not pornography."

The pictures show Ai and four women, all naked, sitting on chairs in the middle of a bare white room.

"Netizens came to take photos with me, so we said, why don't we take nudity photos, then everybody agreed so we did it and they were put on the Internet, and that's it, we forget about it," he said.

"This is completely ridiculous -- our nation today is so corrupt, with so much sex, but they think nudity photos on the Internet is pornography."

Police in Beijing refused to immediately comment on the investigation.

The latest development comes after Ai this week began the process of challenging a bill for 15 million yuan ($2.4 million) in alleged back taxes by paying an 8.45 million yuan guarantee to authorities.

The money was raised from supporters who came from far and wide to help him raise cash, some even throwing banknotes folded into paper airplanes over the walls of his courtyard home.

Ai, who is most famous in China for his work on the "Bird's Nest" Olympic stadium, said earlier this week that the generosity of the Chinese people had made him realise that he was "not alone" in his struggle.

Ai -- who has been banned from leaving Beijing since his release -- denies the government's charge that he evaded taxes for years, insisting it is a politically motivated attempt to silence his vocal rights activism.

The painter, sculptor, architect and activist has long been a thorn in the side of China's Communist authorities.

He is known for tallying the number of children killed in the 2008 Sichuan earthquake -- a hugely sensitive topic as many died in schools that were shoddily built and collapsed onto them, which many blamed on corruption.

Until recently, he had been left relatively unscathed, despite his activism, thanks to his family background -- his father is the late Ai Qing, a poet who was in turn adored, disgraced and rehabilitated by the Communist regime.

The value of his sculptures, photographs and installations has shot up since his detention in April catapulted him into the global spotlight, and last month the influential Art Review magazine named him the most powerful figure in the art world.

Ai has said he currently has little time for his regular art work, but that he considers his fight against the government to be an extension of his artistic endeavours.

"I think this is my artwork. My artwork is about communication and expressing my social concern," he said.