By Michael BristowBBC News, Beijing
No one has seen or heard from him since. The authorities have not commented.
Ai Weiwei has become one of China's most outspoken critics, complaining about a lack of human rights.
The artist was detained on Sunday morning while travelling with an assistant, Jennifer Ng.
The documents of both were checked thoroughly before Ms Ng was allowed to continue on her journey to Hong Kong.
She told the BBC that Ai Weiwei was taken away by border guards.
"I went back to check with the security officers and they said, 'He has other business - you go on the flight on your own'," she said.
A few hours later, more than 40 police officers raided the artist's Beijing studio.
Dozens of items were confiscated, said another assistant, and several people were taken to a nearby police station. They were released a few hours later.
Vocal critic
Ai Weiwei's artwork is known across the world.
He helped design the main stadium used in the Beijing Olympic Games in 2008. It became known as the Bird's Nest because of its intricate steel latticework.
He currently has an exhibition at the Tate Modern gallery in London. It is made up of 100 million porcelain objects made to look like sunflower seeds, a popular snack food in China.
The artist has also become a vocal critic of the Chinese government.
Some of his work has political connotations - he tried to gather the names of every school child who died during the Sichuan earthquake in 2008.
This is a sensitive subject as many schools fell down in the earthquake, leading to claims that they were shoddily built.
The Chinese government is reluctant to talk about this issue and has arrested activists who do.
Ai Weiwei has also lent his support to others who have tackled the authorities.
He turned up outside the Beijing courthouse when fellow artist Wu Yuren went on trial at the end of last year.
He used the opportunity to talk to the foreign media, berating the government for what he believes is a lack of basic rights and freedoms in China.
These activities have brought him to the attention of the authorities.
The 53-year-old was last year prevented from travelling abroad and, in a separate incident, was briefly held under house arrest.
He is under constant surveillance. Miss Ng, his assistant, said the police had visited his compound three times recently.
She said that had led Ai Weiwei to wonder if his next detention was not far away.
