BEIJING - PROMINENT dissident Liu Xiaobo, who co-authored a manifesto calling for sweeping reforms to China's rigid political system, has been arrested for activities allegedly aimed at overthrowing the country's socialist system, state media said on Wednesday.

Liu has been in police custody since they took him away on Dec 8, a day before the political document was released. It gave no other details. Liu's lawyer said he has not been informed officially of the charges.

It marks the highest-profile arrest of a Chinese dissident since human rights activist Hu Jia was detained last year ahead of the Beijing Olympics. Hu was sentenced to 3 1/2 years in prison for sedition last April.

China has always reacted sharply to any challenges to its one-party system, but is also cracking down on any dissent ahead of a gala celebrating the communist regime's 60th anniversary on Oct 1.

According to Xinhua, police have obtained permission from the courts to arrest Liu on suspicion of committing a crime. Mr Mo said this indicates the investigation into Liu's case is still continuing and that he has yet to be formally charged. The next step would be indictment.

Liu, 53, is a former university professor who spent 20 months in jail for joining the 1989 student-led protests in Tiananmen Square.

In his writings, most published only on the Internet, Liu has called for civil rights and political reform, making him subject to routine harassment by authorities.

He was among more than 300 lawyers, writers, scholars and artists who signed 'Charter 08' in December calling for a new constitution guaranteeing human rights, election of public officials, freedom of religion and expression, and an end to the Communist Party's hold over the military, courts and government.

Police detained Liu a day ahead of the charter's release, possibly because they considered him a key organizer, in addition to his role in drafting and revising the document, Mr Mo has said.

The singling out of Liu for prosecution also seems to be an effort to warn others involved in the charter. Other signatories have been called in for talks with police but not arrested. A Peking University law professor, He Weifang, was reassigned to a post in the far western Xinjiang region after signing the document in an apparent rebuke. -- AP