A well-known dissident has been formally arrested after being detained for six months on suspicion of inciting subversion, Chinese state media has reported.

Liu Xiaobo was charged on with "alleged agitation activities" aimed at overthrowing the government and the nation's socialist system, the official Xinhua news agency reported on Wednesday.

He has been in police custody since December 8, the day before the publication of a document that he co-authored, appealing for political reform in China and an end to one-party rule.

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

Violation

Al Jazeera's Melissa Chan, reporting from Beijing said that it was unclear why the Chinese authorities had acted now.

"This seems to be a very problematic response from the Chinese government, it violates international law and it violates Chinese law" Sam Zarifi, Amnesty director

"There are no anniversaries or commemorations coming up that may prompt the Chinese authorities to arrest him."

Liu's lawyer said that he had not been officially informed of the charges.

Sam Zarifi, the director of the Asia-Pacific programme for Amnesty International in London, told Al Jazeera the government is in violation of China's own Criminal Procedure Law.

"This seems to be quite a silly way for the authorities to deal with Liu Xiaobo, who after all doesn't have any real political threat to authorities except for the fact that he has articulated positions regarding human rights," Zarifi said.

"He was kept away from his family and lawyers ... this seems to be a very problematic response from the Chinese government, it violates international law and it violates Chinese law."

Pro-democracy protests

A former university professor, Liu had been jailed before for his involvement in the pro-democracy protests at Tiananmen Square in 1989.

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.

Rights group said many dissidents were held during the Tiananmmen anniversary [Reuters]

China has changed dramatically in the 20 years since the Tiananmen crackdown, with millions lifted out of poverty and the country's economy now the third largest in the world.

But while the country has changed economically, politically the ruling Communist party shows no sign of loosening its grip.

Rights groups say that dozens of dissidents were detained and harassed ahead of the 20th aniversary of the crackdown in early June.

Already-tight security around Tiananmen Square itself was also stepped up with increased numbers of police and plainclothes personnel deployed in and around the symbolic centre of political power in China.

The government has acknowledged that 241 people died as troops moved to evict protesters from Tiananmen Square and surrounding areas, but activists and human rights group believe that thousands may have died.