An exhibition of his art will also open in the German capital in his absence later this month, as protests against his disappearance continue.
International protests against Ai's arrest have been small but steady, while diplomatic efforts are made behind the scenes to try to secure his release.
Around 400 people demonstrated in the central German city of Kassel on Tuesday night, calling for his release. Mayor Jürgen Kaiser supported the move, saying he would ensure a large poster covered in personal messages for Ai would be passed onto the Chinese authorities with a request to deliver it to the artist.
Notes have been posted on bus shelters in Berlin, bearing the question “Where is Weiwei” in large letters, and a phone number. In small text, the notes gives number of the Chinese embassy and suggests people phone and simply ask where the artist is before hanging up.
Artists and other supporters demonstrated last Sunday in Berlin, Hamburg, Munich and other German towns in their contribution to a day of action across the world.
Ai Weiwei was arrested on April 3, since when nothing has been seen or heard from him. The Chinese authorities have issued conflicting statements about why he was seized, as well as criticising the action taken in other countries to attract attention to the arrest.
Various German politicians have condemned his disappearance in public and in letters to the Chinese authorities while others have called for the prestigious exhibition of German art in the Beijing National Museum to be closed.
A debate over what to do will be held on April 26, with politicians and culture administrators taking part. The German Cultural Council has already called for the exhibition to be reconsidered.
The council's manager Olaf Zimmermann said that should Ai not be released soon, an early closure of the exhibition would be “difficult to avoid.”
Now it seems certain that Ai will not be able to attend the opening of his own exhibition on April 29 in Berlin, moves are being made to ensure his absence does not go unnoticed.
“We are very concerned about the well-being of Ai Weiwei, his family and colleagues and friends,” the Neugerriemschneider Gallery said in a statement which also confirmed that the exhibition would open as planned.
Perhaps China's best-known artist, Ai announced plans last month to move to Berlin to escape growing repression in his home country.