French sculptor Daniel Buren has decided to cancel an exhibition scheduled for mid-July in Beijing in \"solidarity\" with detained artist Ai Weiwei, he told AFP. Buren, perhaps best known for his striped columns in the historic state-owned Palais Royal gardens near the Louvre in Paris, was to have shown a vast exhibit at the Ullens Center for Contemporary Art from July 15. \"I don\'t think it is possible to do nothing,\" Buren told AFP. \"It\'s almost an ethical obligation to act,\" added the Frenchman, who was to have headed for Beijing early next month. The outspoken Ai Weiwei, 54, a constant thorn in the side of Chinese authorities, was detained in Beijing on April 3 as he tried to board a flight to Hong Kong. He is under investigation for \"economic crimes\" and relatives of the artist say they do not know where he is. Renowned British sculptor Anish Kapoor said Tuesday he had called off a planned exhibition in China over the ongoing detention of Ai Weiwei. The British Council had asked Kapoor to plan a show at Beijing\'s National Museum of China as part of next year\'s \"UK Now\" festival. Buren said: \"First I signed petitions to free Ai Weiwei. But as he is still being held, virtually in secret, after two months I think the best solution is for me to pull out.\" The artist, who has exhibited several times in China, continued: \"If I carried on without doing anything, it would be a mistake that I would regret for the rest of my life. \"When freedom of expression is flouted in a country, what value can be given to the works of artists who are still allowed to express themselves? That compromises our own work.\" Ai, who helped conceive the \"Bird\'s Nest\" the 2008 Olympic stadium in Beijing, investigated school collapses in the 2008 Sichuan quake and conducted a \"citizen\'s probe\" into a Shanghai fire that killed 58 people last November. His detention came as part of a major government crackdown on dissent by China\'s rulers following anti-government protests sweeping through the Arab world.