Ai was arrested by the Chinese police on 3 April in Beijing as he was about to board a flight for Hong Kong.
Kapoor – who has dedicated his installation at the Monumenta exhibition, which opens on Wednesday at Paris’s Grand Palais, to Ai – told BBC Radio 4′s Today programme that artists needed to look out for each other.
“As a colleague – I don’t know him personally – I feel that as artists we have a communal voice and it’s important that we stick together, that we have a sense of solidarity with each other,” he said.
“It would be nice to see the art world come together a little more. Perhaps all museums and galleries should be closed for a day across the world. I think some such campaign needs to form itself.”
Kapoor said the Chinese state’s behaviour was reminiscent of the Soviet era, when “the voice of artists of different kinds was seen as being dangerous”.
He added: “It does bear witness to the barbarity of governments that if they’re that paranoid they have to put away artists. It’s a ridiculous situation.”
The sculptor added that if the Chinese authorities had a case against Ai, they should bring it as soon as possible.
“Let’s get on with it then – let them accuse him of something,” he said. “It’s a month now that the poor man has been held without a voice. But not only that: his family doesn’t know where he is. This is not a situation that’s acceptable in any circumstances.”