Chinese artist and government critic Ai Weiwei said Friday he was suing Beijing's tax bureau for violating the law when it imposed a multi-million tax evasion fine on a company he founded.

Ai -- who disappeared into custody for 81 days last year as police rounded up dissidents amid online calls for Arab Spring-style protests in China -- has always denied the charge and says it is politically motivated to silence his activism.

He was accused of tax evasion linked to Fake Cultural Development Ltd -- a company he founded but registered to his wife -- upon his release from custody last June.

The Beijing tax bureau subsequently in November issued a bill for 15 million yuan ($2.4 million) in alleged back taxes, giving the artist 15 days to pay it or hand over an 8.45 million yuan guarantee.

Ai was able to pay the guarantee -- needed by law to challenge the charge -- thanks to a huge wave of donations from supporters of his activism and art.

"The whole accusation over FAKE company's tax issues has no basis and it (Beijing tax bureau) violated the law, they didn't have correct procedures," he said, adding his lawyers had filed two lawsuits in two separate Beijing courts.

The 54-year-old told AFP the tax office never allowed the firm's lawyers to review the evidence brought against it and added the bureau never even saw the full, original file against the company, which the police still holds.

He added that the accountant and manager of FAKE are still missing and have not been allowed any contact with Ai and the company's lawyers.

"The tax bureau said it's not their fault because the police somehow forced them to do it (impose the fine), they never saw the originals (of the evidence), they never saw the accountant or manager, they never even questioned them," he said.

"It's a very political order, they have to do it... But the only thing we can do is sue them because they put the charge on us.

"We have no other way to clear our name... All we can do is do this openly."

The artist has previously angered authorities with his investigation into the collapse of schools in the 2008 Sichuan earthquake and into a 2010 fire at a Shanghai high-rise that killed dozens.

His art work has sold worldwide and he was named the world's most powerful art figure by influential British magazine Art Review last year.