Chinese political activist and artist Ai Weiwei says police have prevented him from leaving his Beijing studio to attend a court hearing on his tax evasion case.

Ai Weiwei said that a number of police cars arrived at his studio and an employee filming the vehicles was roughed up.

Tax authorities imposed a $2.4 million fine on Ai Weiwei’s firm for tax evasion in 2011.

Supporters say the fine is politically motivated.

The Chinese authorities maintain that the firm, called Fake Cultural Development, owes them money and it must be paid back.

Ai Weiwei said that a number of police cars arrived at his studio and an employee filming the vehicles was roughed up

While Ai Weiwei is a designer for Fake Cultural Development, his wife is the legal representative of his company. She is believed to be attending the hearing in Beijing.

Ai Weiwei, 54, said on Twitter that the police had destroyed the camera of one of his employees. There were also photos showing injuries that his employee suffered after being roughed up by police.

The artist, a outspoken critic of the government, was detained for almost three months without charge last year. After he was released, he was accused of tax evasion and the fine imposed.

Ai Weiwei has said that the tax bill is pay-back for his activism. He challenged the fine in court, saying proper procedure was not followed.

A Beijing court then agreed to hear the case, a move that caught him by surprise.