Chinese authorities on Tuesday ordered artist Ai Weiwei to pay 15 million yuan ($2.36 million) in alleged back taxes in what the vocal rights activist called an effort to "crush" him.

The tax bill comes after Ai was released in June from 81 days in secretive police custody, a detention authorities said was over tax evasion but during which he said he was repeatedly interrogated over his calls for human rights.

"They gave a written notice today... there was no explanation whatsoever. We questioned where this figure came from -- they couldn't give a clear answer," Ai told AFP.

"The notice said I have 15 days to pay. That's about one million a day... if you don't pay they could put you in jail, maybe up to seven years. I really have no idea."

Ai, 54, a globally renowned conceptual artist who is famous for his part in designing Beijing's "Bird's Nest" Olympic Stadium, said he may be able to appeal against the tax bill, but only after it is paid.

He was seeking to consult lawyers and tax experts, he said.

His mother has offered to sell the house left to the family by Ai's father, the late Ai Qing, a well known poet praised by the Communist Party, but who also suffered political persecution.

Ai called the tax an "injustice" and alleged it was a political move by the government to silence him.

"During the 81 days (in detention) all the police talked about was subversion of state power, so I am very surprised they are avoiding talk about politics, (and now talk) about this tax," Ai said.

"This is a signal that the state can seize anybody who has a different political opinion. They use tax or whatever reason to make them look bad or to crush them."

Police who took Ai into custody early this year met with the artist on Monday and urged him to abide by the tax ruling, he said.

"They said, 'If the state says you have a problem, you have a problem. You can't argue, the state will never change its mind'," he said.

Ai said the tax was being charged to him as "the actual controller" of the Beijing FAKE Cultural Development Ltd, a company where he works but which is owned by his wife.

Ai, whose detention sparked outrage around the world, was released in June in a move authorities said was due to his "good attitude" in admitting to the charges against him, his willingness to repay taxes he owes and on medical grounds. He has diabetes.

London and Berlin were especially vocal about the case. Ai's work has been displayed this year at the Tate Modern gallery in the British capital, and the University of the Arts in Berlin has invited him to be a guest professor.

Ai's problems with the 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.

He says police beat him when he tried to testify on behalf of another activist in 2009.

His detention in April came as scores of activists and rights lawyers were rounded up by police amid anonymous calls on the Internet for street protests in China similar to those that toppled governments in the Arab world.

After his release, Ai was warned not to publicly discuss his case or accept interviews from foreign journalists.

Last month Ai was named the world's most powerful art figure by influential British magazine Art Review.