Beijing - Liu Xiaobo, a prominent Chinese human rights activist, has been detained by police for "undermining the state's authority," Deutsche Presse-Agentur dpa was told Wednesday by a lawyer supporting Liu.

Liu's arrest - on the eve of the 60th anniversary of the declaration of universal human rights - prompted strong international condemnation and reactions by rights organizations worldwide.

The writer and chairman of China's Pen Club was detained late on Monday after signing "Charta 2008," a joint appeal for more democracy and freedom in China by about 300 intellectuals and activists on the occasion of the anniversary of the 1948 human rights declaration.

The circumstances of Liu's arrest, with his apartment searched and his computer seized, indicate that the former philosophy professor at Beijing University faces prosecution, sources close to Liu said.

Liu, 52, has been a high-profile critic of the Communist government in China for two decades. The activist group Human Rights in China said police presented a formal writ at his arrest, accusing Liu of "undermining the state's authority."

Another human rights activist, Zhang Zhuhua, was also detained but later released after 12 hours of questioning, the group said in a statement. Chinese security forces also ransacked the homes of the two men, and seized computers, books and personal items, they said.

"By detaining these individuals on the eve of the international Human Rights Day, the Chinese government has exposed the emptiness of the rhetoric in its recently announced Human Rights Action Plan," the organization said.

The arrest "makes a mockery of the official slogan that people are the foundation of China's development and of the official claim that 'human rights work in China has already achieved historic progress'," they said.

Beijing's move against Liu and Zhang followed the arrest, earlier this month, of a number of other dissidents calling for more freedom of opinion and of the media, the organization Reporters Without Borders said Wednesday.

Charta 2008 echoes the renowned Prague Charta 77, in which dissidents including later Czech president Vaclav Havel slammed rights violations in the former communist-ruled Czechoslovakia in January 1977.

The Chinese declaration presents specific recommendations to improve human rights, including constitutional reform, the separation of administrative, legislative and judicial powers, freedom of association, expression, and religion, as well as a public campaign on civil rights, according to Human Rights in China.

Among the signatories was Ding Zilin, a retired professor, who heads the network of families affected by the 1989 Tiananmen Square massacre, as well as rights activist Yao Lifa and Zeng Jinyan, the wife of detained civil rights activist Hu Jia.

"This kind of reaction to the legitimate appeals of Chinese citizens to the government sends an unsettling message that a crackdown for next year may be already underway," Human Rights in China commented on the arrests.

In 2009, when the People's Republic of China turns 60, Beijing also will see the 20th anniversary of the Tiananmen Square massacre in which hundreds of civilians were shot dead by Chinese security forces. A further sensitive date is the 50th anniversary of the exile of Tibetan spirtual leader, the Dalai Lama.