Chinese authorities have served Ai Weiwei with an official demand telling him to pay 15m yuan ($2.3m; £1.4m) within 15 days, the artist has said.He said he had rejected the notice, and was not sure whether he would pay.The artist, one of China\'s most famous people, was held for almost three months earlier this year before being accused of \"economic crimes\".His supporters say the accusations are part of a plot to silence Mr Ai, who is an outspoken critic of the government.Mr Ai told Reuters news agency he would pay the money if it was proved to be a tax issue.But he said he had not been able to review his company\'s account books because they had been taken by the authorities.And he said he had not seen any evidence showing that the firm had evaded tax.\"If it\'s a tax problem, I\'ll pay. But if it\'s not, I won\'t pay. This whole matter is ridiculous,\" he said.Mr Ai was picked up by police in April as authorities rounded-up activists, following calls on websites for a Middle Eastern-style Jasmine revolution in China.The state news agency Xinhua said in June that Mr Ai had been released \"because of his good attitude in confessing\" to tax evasion and because he had agreed to pay back the money he owed.Since his incarceration, the artist has become a cause celebre for rights activists and critics of China\'s Communist Party.He has won numerous art awards, and was recently named the world\'s most powerful artist in a poll carried out by an art magazine.