Chinese artist Ai Weiwei says he has been stopped from travelling abroad, in a move apparently linked to next week's Nobel Peace Prize awards in Norway.
Ai Weiwei, a fierce critic of Beijing, said he was due to travel to South Korea, but police told him his trip could "threaten national security".
The authorities have reportedly been working to prevent activists from attending the Nobel ceremony.
The foundation is giving its prize to imprisoned campaigner Liu Xiaobo.
Since the Nobel announcement in October, Mr Liu's wife and other prominent dissidents have either been put under house arrest or are under increased surveillance, says the BBC's Martin Patience in Beijing.
Last month, a Nobel official said that the award may not be handed out at the ceremony because none of Mr Liu's close relatives were expected to attend.
Ai Weiwei, one of China's most famous artists, described the decision to stop him leaving the country as "really silly".
"I think there's a direct connection with next week's Nobel Peace Prize award - the Chinese government is very upset about this," he told journalists.
Another supporter of Mr Liu, economist Mao Yushi, was also stopped from leaving the country this week on grounds of national security.
In a web message reported by the Associated Press, he said: "It reminded me of the Cultural Revolution."
Neither man has been invited to next Friday's Oslo ceremony, and both have said they have no intention of going.
Chinese officials have not commented on their claims.
Mr Liu, 54, was a leader in the Tiananmen Square protests in 1989. Last year, he was sent to jail for 11 years for authoring a document calling for a multi-party democracy.
The Nobel Foundation described him as the "foremost symbol" of the struggle for human rights in China, but Beijing denounced him as a criminal.
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