BEIJING - Outspoken Chinese artist Ai Weiwei said Friday that police have begun investigating his assistant for allegedly spreading pornography online, following weeks of campaigning by the dissident against a $2.4 million tax bill he says is harassment.

Police called in videographer Zhao Zhao, one of Ai's assistants, for questioning on Thursday, Ai told The Associated Press by phone.

Zhao said police asked him about an artistic photo he took last year of Ai and four women, who were all nude, titled "One Tiger, Eight Breasts."

The investigation appears to revive an accusation levelled against Ai when he was detained in April. Reports then said Ai was being investigated for tax evasion, bigamy and for spreading pornography online.

Ai questioned whether the police understood art and culture. "If they see nudity as pornography, then China is still in the Qing dynasty," he said.

Beijing's public security bureau did not immediately respond to questions faxed to them.

Ai, an internationally acclaimed conceptual artist, was detained for nearly three months earlier this year during an overall crackdown on dissent. The detention and subsequent claims of tax evasion have been interpreted by activists as a way to punish him for his outspoken criticism of the authoritarian government.

The photograph features the hefty, bearded artist and four women sitting on traditional Chinese wooden chairs on a concrete floor against a bare, white wall.

Zhao said police told him they might seek criminal penalties against him for the photograph if it had spread widely.

"They said, 'This photograph is an obscene photograph.' I said, 'I didn't know that, what's obscene about it?' They said, 'It's just obscene.' They've already decided that," Zhao said. No charges have been pressed yet, he added.

Zhao said the investigation at this point appeared to be limited to himself, but that he believed it was part of the authorities' ongoing campaign against Ai.

"To them so far their efforts have had no effect, such as the tax case, so they are trying this from other angles," he said. "They are again raising this issue, and will keep repeating this over again. They won't easily let him off."

The Beijing tax bureau is demanding that Ai's design company Beijing Fake Cultural Development Ltd. pay 15 million yuan ($2.4 million) in back taxes and fines. This week, the artist paid a $1.3 million guarantee into a government bank account in order to contest the bill.