On Friday, with no sign that the Chinese government was planning to free artist Ai Weiwei from custody anytime soon, the Tate Modern in London offered up a simple message on its Northern facade, sending an unambiguous message as to where it stood on the matter: "Release Ai Weiwei."

The artist, of course, has been showing his epic "Sunflower Seeds" installation in the Tate's Turbine Hall since October. In a note appended to the online announcement for that work, the institution stated, "We understand from news reports that the artist Ai Weiwei was arrested by the Chinese authorities yesterday as he tried to board a plane to Hong Kong. The artist remains uncontactable and his whereabouts are unknown. We are dismayed by developments that again threaten Weiwei's right to speak freely as an artist and hope that he will be released immediately."

The Guggenheim Foundation made a gesture of its own, launching a bilingual online petition addressed to Chinese culture minster Cai Wu, rallying the leaders of the world's preeminent cultural institutions to add their voices to the call to free Ai Weiwei. The statement made clear that the signatories hoped "to hasten the release of our visionary friend."

In addition to Guggenheim director Richard Armstrong and Guggenheim Asian art curator Alexandra Munroe, the names on the list included LACMA director Michael Govan; Association of Art Museum Directors president and Minneapolis Institute of Arts head Kaywin Feldman; Museum of Modern Art director Glenn Lowry; Gwangju Biennale Foundation president Yongwoo Lee; Asia Society president Vishakha Desai and Asia Society director Melissa Chiu; Tate director Sir Nicholas Serota; and new Tate Modern chief Chris Dercon.

This morning, a letter published in the Guardian, spearheaded by curator and lecturer Katie Hill, featured the signatures of hundreds of art professionals. It urged "the UK government to treat this case as an urgent priority and to press for his immediate release." The list of signatories encompasses a broad and eclectic cross-section of the international art world, including artists Jeremy Deller, Nicole Eisenman, Tehching Hsieh, Isaac Julien, Roman Ondak, and Rob and Roberta Smith, critics Adrian Searle and Griselda Pollock, Whitechapel director Iwona Blazwick, and many others.

Of the courageous artists actually based in China whose names appear on the Guardian letter, the most notable are the Gao Brothers and Cao Fei.