Chinese artist and activist Ai Weiwei, seen outside his demolished Shanghai art studio in January, has not been heard from since his arrest at Beijing airport on April 3. (AFT/Getty)

Supporters of Chinese artist Ai Weiwei, arrested April 3 by his government, staged a solemn demonstration in London, England to demand his release.

A group of artists read out quotes from Ai on Wednesday just ahead of the opening of two exhibits of his work in the British capital this week.

They called on the Chinese government to free the artist, whose whereabouts are unknown since he was arrested at Beijing airport. The Chinese government says he faces charges of tax evasion and producing "obscene art."

Ai had been keeping an informal tally of the recent detentions of activists, lawyers and writers in China on Twitter. China has stepped up those detentions since February when online calls for protests similar to those in the Middle East and North Africa began to circulate.

Ai Weiwei's 12 massive bronze animals, in the courtyard of Somerset House in London, are a reference to the Chinese zodiac. (Matt Dunham/Associated Press)

Ai, an internationally renowned artist, made headlines earlier this when he placed one million hand-made sunflower seeds on the floor of London's Tate Modern. That exhibit just closed over the weekend.

Demonstrators on Wednesday held their protest in the courtyard of Somerset House where 12 giant animal heads created by Ai stand in a circle.

Gwyn Miles, the director of Somerset House Trust, said Circle of Animals/Zodiac Heads was planned two years ago, "but doing it now shows support to Ai Weiwei."

"Like everyone who admires and respects the work of Ai Weiwei, we are very upset by the news that he has been detained by the Chinese authorities."

Each of the heads are re-creations of the traditional Chinese zodiac sculptures which once adorned the fountain clock of Yuanming Yuan, an imperial retreat outside Beijing, and were pillaged by French and British troops in 1860.

'Where is Ai Weiwei?'

Writer Ekow Eshun, who organized the protest, said it was important to keep Ai Weiwei in the public eye.

"Everyone in the Western world is united to have his release. The most important thing is to show his work and to keep asking the question: 'Where is Ai Weiwei?'"

Another Ai Weiwei show at the Lisson Gallery in London launches on Friday. Visitors will be invited to be photographed with a "free Ai Weiwei" sign with the pictures posted online to go with a petition.

Artists, human rights activists and journalists around the world have been keeping Ai Weiwei in the spotlight. In Paris, a gigantic inflatable plastic sculpture by British artist Anish Kapoor, called Leviathan, has been dedicated to Ai.

Kapoor suggested that museums around the world host a one-day dedication to Ai: "[His detention] takes us back to a Soviet-style time when the voice of artists, of different kinds, are seen as being dangerous."

Academics in Germany have made him a visiting lecturer at Berlin University, bestowing him honorary membership of the city's academy of the arts.

In Hong Kong, Ai's portrait has appeared at the city's military garrison.