BEIJING — In the days since the Chinese government delivered a punitive $2.4 million tax bill to the artist Ai Weiwei, thousands of people have responded by contributing money in a gesture that is at once benevolent and subversive.

More than 20,000 people have together contributed at least $840,000 since Tuesday, when tax officials gave Mr. Ai 15 days to come up with an amount that was more than three times the sum he was accused of evading in taxes.

“It’s surprising; it has really changed my perspective on people,” he said in a telephone interview on Sunday, describing how scores of supporters, some of whom traveled from distant cities, have been delivering cash to his home.

One of China’s best known artists and a voluble government critic, Mr. Ai was detained in April and held for 81 days at an undisclosed location, ostensibly on tax evasion charges, according to the state-run news media. Mr. Ai insists his prosecution is politically motivated.