Chinese artist Ai Weiwei said Friday he will challenge a huge tax bill the government has ordered him to pay, and will use money collected from supporters to lodge an appeal.

Ai, who disappeared into police custody for 81 days earlier this year and was ordered to pay 15 million yuan ($2.4 million) in back taxes after his release, needs to pay the bill by Tuesday.

But the artist, best known for his role in designing Beijing's "Bird's Nest" Olympic stadium, said he will contest the order and will use funds donated by supporters as a guarantee to make an appeal -- a step required by Chinese law.

"We never said we will pay the tax bill," Ai told AFP on Friday.

Ai said he had planned to use his mother's home as collateral for the appeal, but that will "take much longer" than anticipated to organise so he had decided to use money from his supporters.

The 54-year-old said previously that 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 has denied any wrongdoing and insists the government is trying to silence him and his vocal human rights activism.

Pu Zhiqiang, a lawyer acting for Ai, told AFP on Thursday that a guarantee of eight million yuan needed to be paid in order "to pursue the request of administrative revision".

The drive to donate to Ai has gathered momentum since beginning last Friday, with supporters coming from as far afield as Hainan island in the south, 3,000 kilometres (1,860 miles) away, to give money.

Supporters have been sending Ai money through Internet and bank transfers, while some have even resorted to throwing cash over the walls into his courtyard home, including banknotes folded into paper planes.

Total donations had reached 6.7 million yuan, Liu Yanping, who works with Ai, said on her Twitter account on Thursday.

Ai, whose artworks have sold worldwide -- some reportedly for hundreds of thousands of dollars -- has acknowledged previously that he does not need the financial support.

"What I need is the ethical support of everybody. I don't need the money," he said.

He has vowed to pay back the money to his donors, some of whom are prominent activists.

Ai's activism has incensed the government in the past. He has organised independent investigations into the collapse of schools in the 2008 Sichuan earthquake and into a 2010 fire at a Shanghai high-rise that killed dozens.