Prominent Chinese artist and activist Ai Weiwei has been stopped by the authorities from leaving the Chinese capital, Beijing, reports say.
There has been no contact with him since, and no comment from the police. One of his assistants told the BBC he had been held by border guards as he tried to get on a flight to Hong Kong.
She says police have searched his Beijing home, which is also his studio.
More than 20 dissidents and activists have been held in the past weeks.
China's authorities appear on edge over calls for a so-called Jasmine Revolution, partly inspired by pro-democracy movements in the Middle East.
Ai Weiwei's compound in Beijing has now been cordoned off.
It is not clear whether the 53-year-old artist has been detained or why he was barred from taking the flight from Beijing Capital International Airport.
A number of people who work for him were apparently taken to a local police station, but there has been no word from the authorities or the artist himself.
Ai Weiwei is a controversial figure in China, says the BBC's Michael Bristow in Beijing. He is a successful artist with an international reputation, but he has used his fame and position to criticise the Chinese government about human rights.
That has brought him to the attention of China's security forces, who have been monitoring him for some time, our correspondent adds.
They have previously restricted his movements.
He was stopped from boarding a flight to Seoul last November, when police told him he could cause damage to national security by leaving, and in another incident briefly placed under house arrest.
Born in 1957 in Beijing, Ai Weiwei has played a key role in contemporary Chinese art over the last two decades, and helped create the Olympic "Bird's Nest" stadium in his home city.