Chinese intellectuals urge political activist's release

Many of the 50 signatories to the open letter also put their names to Charter ‘08, a document demanding democracy and other freedoms co-authored by Dr Liu last year and believed to have prompted police to detain him.

“We appeal for the immediate release of Dr Liu Xiaobo and the real implementation of the freedom enshrined in the constitution,” the letter says.

"Liu Xiaobo's troubles show the environment each of us must survive in ... He cannot freely express his views, and that means that none of us can freely and openly express our views."

Among those who signed it are leading sociologist Xu Youyu, elderly economist Mao Yushi and veteran lawyer Zhang Sizhi, who defended Jiang Qing - Mao Zedong's widow and leader of the so-called Gang of Four.

Liu, who had been held by police at a secret location for more than six months, was formally arrested this week on suspicion of "inciting to subvert state power," a charge that carries a maximum sentence of 15 years in jail. He is currently being held at a police detention center in Beijing.