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One of China's best-known artists, Ai Weiwei, says he has been under house arrest at his home in Beijing.

He says the authorities want to prevent him holding a party to mark the forced demolition of his new Shanghai studio.

Mr Ai was initially invited to build the space, but it has now been declared illegal and will shortly be demolished.

The artist helped create Beijing's Olympic Bird's Nest stadium and his latest installation of sunflower seeds is showing at London's Tate Modern.

Born in 1957 in Beijing, the artist has played a key role in contemporary Chinese art over the last two decades, and has been highly vocal about human rights issues in the country.

Unmarked van

Mr Ai had planned to hold a party at his $1.1m (£670,000) Shanghai studio on Sunday prior to its demolition.

But on Friday, he said that men he suspected were plain-clothes police officers told him he would not be allowed to travel to Shanghai.

A van without numberplates with more than 10 men inside was blocking the exit from his home at an artists' colony in Beijing, he added.

"I'm under house arrest to prevent me from going to Shanghai. You can never really argue with this government," Mr Ai told the Associated Press by telephone.

According to messages on his Twitter feed, Mr Ai has been told he will be under house arrest until midnight on Sunday.

"Please accept my deepest apologies," he tweeted to his guests in Shanghai.

He said that some people still planned to travel to his studio.

He had planned to serve river crabs at the event.

It is thought that the choice of crabs was a political statement as the Chinese name for river crab sounds like "harmonise", a euphemism often used by the Chinese authorities for censorship.

Shanghai had imposed a six-month moratorium on large-scale building and demolition projects during the World Expo, in a bid to improve air quality, but reports say these have resumed since the exhibition ended.

"Ai's studio did not go through the application procedures, therefore, it is an illegal building," Chen Jie, director of the urban construction department in Malu township, where the studio is located, is quoted as telling the .

Mr Ai's latest work is currently on display in London's Tate Modern gallery: a giant installation made up of millions of tiny replica sunflower seeds.

Sunflower seeds are a popular Chinese street snack but also hold another meaning for the artist.

During the Cultural Revolution, propaganda images showed Chairman Mao as the sun and the mass of people as sunflowers turning towards him.

"The seed is a household object but at the same time it is a revolutionary symbol," Mr Ai has said of his work.