Activist groups immediately condemned the move, calling the arrest of the internationally known dissident writer a blow for human rights and political freedom in China.

Liu, who has been detained since December, was arrested Tuesday for "alleged agitation activities aimed at subversion of the government and overthrowing the socialist system," Xinhua news agency said, citing Beijing police.

The 53-year-old writer, who was involved in the 1989 Tiananmen Square pro-democracy movement that was crushed by the army, has long campaigned through his writings for human rights and democracy in China.

He was taken into custody last year after signing Charter 08, a widely circulated manifesto calling for fundamental political and legal reforms to China's Communist Party-dominated system and respect for human rights.

The document was signed by about 300 Chinese scholars, lawyers and officials.

"Liu has been engaged in agitation activities, such as spreading of rumors and defaming of the government, aimed at subversion of the state and overthrowing the socialist system in recent years," Xinhua quoted a police statement as saying.

Liu had confessed to the charge, the statement added.

His lawyer, Mo Shaoping, said he was not informed of the arrest, but that police were still in the process of investigating the case. If formally charged Liu could get up to 15 years in prison, Mo said.

Mo said police had told Liu's wife, Liu Xia, that he might not be able to work as Liu's defense lawyer. Mo said this may be because he also had signed Charter 08.

Liu has been under a form of house arrest in an undisclosed location away from his home since December. Nicholas Bequelin, a Hong Kong-based researcher with Human Rights Watch, said the arrest meant Liu would almost certainly go to prison, calling it "a pre-determined result."

"We believe that Liu Xiaobo has done nothing but exercise his right to freedom of expression and in no circumstances should he have been arrested," Bequelin said.