Chinese artist Ai Weiwei is shown at his Beijing home on April 10, 2009. He was prevented from leaving China on Thursday after Chinese authorities said his travel could 'harm national security.' (Frederick Brown/AFP/Getty Images)Chinese artist Ai Weiwei is shown at his Beijing home on April 10, 2009. He was prevented from leaving China on Thursday after Chinese authorities said his travel could 'harm national security.' (Frederick Brown/AFP/Getty Images)

Chinese artist and government critic Ai Weiwei has been prevented from leaving China in a move that may be linked to next week's Nobel Peace Prize ceremony.

Ai, 53, told Agence France-Presse he had cleared customs and was waiting to board a flight to South Korea on Thursday when two border control policemen pulled him aside.

They handed him a note from the Public Security Bureau that said it could "harm national security" if he is allowed to leave China, Ai said.

Ai, a successful artist whose work was exhibited recently at the Tate in London, has been a critic of the government for suppressing free speech and for its treatment of earthquake victims and their families.

Ai said he believed the restriction was linked to the Nobel ceremony due to take place on Dec. 10 in Oslo. Jailed dissident Liu Xiaobo has been awarded the peace prize and China wants to stop another activist from accepting on his behalf.

Scores of activists and lawyers have been prevented from leaving China in recent weeks in what is widely seen as a crackdown linked to the prize.

China has accused the Nobel Prize committee of interfering in its affairs by awarding the prize to Liu, who is serving 11 years for subversion after co-authoring a petition calling for democratic reform. He also was an activist in 1989, during the Tiananmen Square pro-democracy protests.

The Nobel Foundation described him as the "foremost symbol" of the struggle for human rights in China, but Beijing has denounced him as a criminal.

Liu's wife is under house arrest and 80-year-old economist Mao Yushi has said he was prevented from flying to Singapore on Wednesday morning because of his links to Liu.

Ai said he was going to South Korea, and then planned to travel to Germany, Ukraine and Denmark, but had no plans to go to Oslo.

Beijing has also tried to persuade or pressure foreign governments to boycott the Nobel award ceremony.

Some Chinese dissidents living outside the country have plans to attend, among them Wan Yanhai, an AIDS advocacy activist who fled to the U.S. this year.

With files from The Associated Press