BEIJING — Chinese authorities have extended the detention of a jailed dissident writer for another two months, his lawyer said Wednesday.

Police took Liu Xiaobo away on Dec. 8, 2008, one day before the publication of a document he co-authored that called for more civil rights in China and an end to the Communist Party's political dominance.

Liu was held at a secret location for six months, then formally arrested in June on suspicion of "inciting to subvert state power" — a loosely defined charge that carries a maximum sentence of 15 years.

Lawyer Mo Shaoping said police gave no reason for Wednesday's extension other than that the case was complicated.

Under Chinese law, no more extensions are allowed without issuing an indictment and sending the case to trial.

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

He was one of the chief architects of "Charter 08," an unusually direct call for a new constitution guaranteeing human rights, the open election of public officials, and freedom of religion and expression. The document, released in December 2008, demanded an end to the Communist Party's overarching control of the military, courts and government bodies, and a new criminal code eliminating overtly political crimes such as incitement to subvert state power.

More than 300 lawyers, writers, artists and intellectuals signed the charter when it was released, although Liu is the only one who has been arrested — seen as a move by authorities to scare other signatories into backing off.

China's communist leaders have tolerated no political challenges to their authority since the crushing of the 1989 protests by the military. Most dissidents have been harassed into inactivity, imprisoned or exiled. Hopes that last year's Beijing Olympic Games would foster greater moderation proved unfounded.