Supporters of Chinese artist Ai Weiwei have contributed nearly 1.8 million yuan ($285,000) towards a huge tax fine imposed after he was released from detention this year, a friend said on Saturday.

Ai, whose 81-day disappearance into police custody caused an international uproar, was this week ordered to pay 15 million yuan ($2.3 million) in back taxes by mid-November, prompting supporters to issue an appeal to help him out.

The artist, best known for his role in designing Beijing's "Bird's Nest" Olympic stadium, has denied any wrongdoing and insisted the government is trying to silence him and his vocal human rights activism.

Ai told AFP on Friday that more than 10,000 people, mostly students and young people, had sent in donations to help pay the fine.

By early Saturday, the total raised had reached nearly 1.8 million yuan, Zuoxiao Zuzhou, a rock musician and friend of Ai's said on his microblog.

He said contributions had been pouring in via money orders, online payments, cash and bank transfers.

Ai used his Twitter-like Weibo account to thank some supporters individually. "What counts is not the amount (of each contribution) but the number of people involved," he wrote.

He told AFP on Friday that he was overwhelmed by the response.

"It's unbelievable. They were saying the contribution was like a vote, that fining Ai Weiwei was like fining them."

Ai said he had vowed to record and eventually return every donation, although officials have already told him not to accept the funds.

Ai's troubles with the authorities began after he launched 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.

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.

Upon his release in June, the government charged him with tax evasion.

Prominent democracy activist Ai Xiaoming, a professor at Zhongshan University in south China's Guangdong province, told AFP she had so far helped raise 400,000 yuan for the artist.

Hu Jia, another famous rights activist who was released from prison earlier this year, said in a Twitter posting he donated 1,000 yuan to Ai for "my great gratitude and respect for what he has done".

This "tax fine actually has nothing to do with the tax bureau, it is really a means used by Beijing police, state security police and (the Communist Party's) law committee to persecute Ai Weiwei," Hu said.

Domestic microblogs have censored the artist's name, and searches of Ai's name on China's wider Internet only turn up official government condemnations of the activist.