Outdoor sculpture exhibition, Circle of Animals/Zodiac Heads, debuted in artist Ai Weiwei's absence [Reuters]

Twelve large bronze animal heads have made their debut in New York City at the absence of their author Ai Weiwei, an imprisoned Chinese artist.

The 53-year-old artist was due to attend Wednesday's opening of the exhibition called Circle of Animals until he disappeared into police custody at Beijing's international airport on April 3.

The Chinese foreign ministry has said Ai is being investigated for "economic crimes", but have not given any specific details. Weiwei has not been heard from since.

Michael Bloomberg, the New York City mayor, called Weiwei's detainment "troubling".

"Artists risk everything to create. They risk failure. They risk rejection. They risk public criticism," he said at Wednesday's unveiling in Central Park where the statues are displayed.

"But artists like Ai Weiwei, who come from places that do not value and protect free speech, risk even more than that."

The exhibit, Circle of Animals, is made up of 12 bronze statues of animal heads representing the 12 personalities of the Chinese Zodiac.

Each of the four-foot-tall animal heads weighs about 363kg and stands atop large marble bases.

The Circle, which was inspired by the design of an 18th century imperial retreat near Beijing, will be displayed in New York until June 26 before going to London, Los Angeles, Houston, Pittsburgh and Washington DC.

Ai is among China's internationally renowned artists. He is best known for his 1979 protests for artistic freedom, provocative works in the 1990s, and in designing the Bird's Nest stadium for the 2008 Beijing Olympics.