Sculptor Anish Kapoor has condemned China's "barbaric" detention of artist Ai Weiwei and has called on galleries to close for a day in protest.
Ai - an outspoken critic of China's human rights record - has not been heard from since he was arrested in Beijing at the start of April.
Kapoor has dedicated his monumental Leviathan art installation in Paris, unveiled on Tuesday, to Ai.
Artists must speak with "a communal voice", he told BBC Radio 4's Today.
China says it is investigating Ai, 53 - who was detained at Beijing airport while passing through security checks for a flight to Hong Kong - for as yet unspecified economic crimes.
His sister, Gao Ge, told BBC News she had not heard anything from her brother and had been given no information as to where he was being detained or whether he had been charged with any offence.
"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," said Kapoor.
"This is not a situation that is acceptable in any circumstances.
"It does bear witness to the barbarity of governments if they're that paranoid that they have to put away artists. It's a ridiculous situation."
The Turner Prize-winning artist said that, while some European foreign ministers had voiced concerns, more must be done.
Kapoor has also called on the art world to unite, adding: "Perhaps museums and galleries across the world should be closed for one day."
Kapoor has described his epic new installation, which has been unveiled in the cavernous nave of the Grand Palais in Paris for the Monumenta 2011 exhibition, as a "single object, a single form, a single colour".
Meanwhile, two major exhibitions of Ai's work will open in London this week.
Somerset House will display his Circle of Animals/Zodiac Heads in its courtyard from Thursday until 26 June.
The outdoor installation comprises 12 monumental bronze animal heads, which are recreations of traditional Chinese zodiac sculptures.
From Friday, the Lisson Gallery is also exhibiting some of Ai's key works created over the past six years.
Lisson Gallery director Greg Hilty has hailed Ai as "one of the leading cultural figures of his generation" who "consistently displays great courage in placing himself at risk to affect social change through his art".
"He serves as an example for legitimate social criticism and free expression both in China and internationally," he added.
A Foreign and Commonwealth office spokesman said the British government was "deeply concerned" by the arrests and disappearances in China of activists, artists, journalists and "others who dare to exercise their internationally recognised rights to freedom of expression".
"These cases are calling into question China's commitment to international human rights standards," he added.