Chinese artist Ai Weiwei said Wednesday supporters had travelled thousands of kilometres to his Beijing studio to express their support and donate money towards a huge tax bill he has to pay.

The drive to donate to Ai, who disappeared into police custody for 81 days earlier this year and was ordered to pay a 15 million yuan ($2.4 million) bill after his release, has gathered momentum since beginning Monday.

The 54-year-old needs to pay the bill by Tuesday, and his studio has been besieged by supporters from morning until evening, some coming from as far afield as Hainan island in the south, 3,000 kilometres (1,860 miles) away.

"People have come from Hebei (north China), Hainan and Fujian (southeast) -- all kinds of people from students to businessmen. They want to show their solidarity," Ai told AFP.

On Wednesday, a stream of donors arrived at the studio, one holding a large bunch of flowers for Ai and another wearing a sweatshirt adorned with the logo "Let's go to find freedom."

The bearded artist himself came in and out of his studio, occasionally posing for pictures in the bustling courtyard while inside several workers sorted through envelopes of donated cash. There was no visible police presence outside.

Ai was charged with tax evasion on his release from detention in June, but he denies any wrongdoing and says authorities are trying to silence his vocal rights activism by imposing the tax bill.

The popular artist was detained in April at the same time as scores of activists were rounded up amid anonymous calls on the Internet for street protests in China similar to those that toppled governments in the Arab world.

Since Friday, 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 have now reached 6.6 million yuan, Liu Yanping, who works with Ai, said on her Twitter account. She told AFP earlier that support for the artist showed no sign of abating.

"We normally open the office at nine in the morning, but there are already people here at eight until 10 in the evening," she said.

The artist said that some of those who did not have much money gave just one yuan (16 US cents).

"They want to tell authorities they are there for support, not for money," he said.

"I feel very surprised, because people say the Chinese never really come together, but in a case like this, where I'm supposed to be someone who violates the law, there is lots of support."

Ai said his mother planned to mortgage her house to help him foot the bill, and that he would be able to pay up by next Tuesday.

The state-run Global Times newspaper suggested Monday in an editorial that experts could charge Ai with "illegal fundraising" for accepting contributions for the tax bill.

Foreign ministry spokesman Hong Lei said Wednesday that "no matter what action the person concerned is taking, it cannot change the fact that Ai Weiwei has evaded lots of tax".

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.