Leading British artists and curators have stepped up pressure on the Bangladeshi government to release the crusading photographer Shahidul Alam from jail in Dhaka.

Creative voices, including the film-maker and artist Steve McQueen, the dancer and choreographer Akram Khan, and the artists Antony Gormley and Anish Kapoor, have joined an international call for justice and transparency about Alam’s alleged crimes, ahead of a bail hearing scheduled for 3 September. A British exhibition of his work is also being planned in support of the cause.

On 5 August the 63-year-old, who launched his career in Britain, was abducted by more than 30 members of the Dhaka metropolitan police and arrested for damaging “the image of the nation”. The arrest follows the photographer’s vocal support for student protesters in the city, many of whom were also arrested after taking part in a demonstration in response to the killing of two students by a speeding bus.

An open letter written by his niece, the architect Sofia Karim, who lives in Britain, has garnered signatures from 47 leading names in the art world in just a week. “Alam’s crime, we are told, is to have contravened the Information and Communication Technology Act. Described as ‘draconian’ by Human Rights Watch, the act has become an infamous means of clamping down on freedom of expression in Bangladesh,” the letter reads. “Given that Bangladesh presents itself as a democracy, the state should respect the right of Dr Alam, and all other citizens, to freedom of expression. Instead, he has suffered inhumane treatment at the hands of the police and judicial system.”

Leading curators, such as the Tate’s Frances Morris, Nicholas Cullinan, the director of London’s National Portrait Gallery, Sarah Munro, director of Gateshead’s Baltic Centre for Contemporary Art, and Sophie Wright of Magnum Photos, have also put their names behind the cause. “Freedom of speech, artistic expression and talking truth to power are vital barometers of civilisation. Whatever is done to one person against these central pillars of a fair and tolerant society is done to all of us,” said Cullinan.

“I always felt that artists would sign the letter, but it’s been very heartening that leaders of our largest cultural institutions have also publicly supported my uncle so warmly, simply from our personal appeals,” said Karim.

The morning after the arrest, Alam was produced in court, shouting that he had been assaulted and threatened with further violence. He was initially remanded for seven days, but then sent to prison before the week was up; neither Alam nor his lawyer was informed or called before the judge.

Alam’s photography focuses on exposing abuses of power, including images of the genocide of the 1971 Bangladeshi war of liberation. He has also chronicled the use of state death squads and the plight of the Rohingya refugees. He founded the picture agencies Drik and Majority World, and the photography school Pathshala South Asian Media Institute.

“When they told my uncle in jail that exhibitions around the world were being held for him, he smiled and said, ‘A big thank you’,” said Karim. “He is in great physical and mental pain at the moment.”