Mr Liu has not been heard of since his arrest in early December
More than 150 writers and activists from around the world have written an open letter to China demanding that a prominent academic be freed from jail.
Liu Xiaobo was arrested earlier this month for signing a petition calling for democracy in China.
Among those demanding his release are British writer Salman Rushdie and Italian novelist Umberto Eco.
In response, China's foreign ministry said the country was under the rule of law and opposed foreign interference.
The letter sent to Chinese President Hu Jintao was signed by a range of Chinese and international lawyers, Nobel prize winners, academics, writers and rights advocates.
In it, they expressed their "deep concern" with the ongoing "arbitrary detention" of Mr Liu.
Nothing has been heard of the former professor since he was detained by police on 8 December.
The writers said the timing led them to presume that Mr Liu was arrested "solely for exercising his right to freedom of expression, as guaranteed under China's constitution and international law".
Mr Liu was one of more than 300 prominent Chinese intellectuals to sign an online petition in December called Charter 08 in December.
Released to coincide with the 60th anniversary of the Universal Declaration of Human Rights, the charter called for greater freedoms and democratic reforms in China, including an end to Communist one-party rule.
The letter writers said Mr Liu's activities had always been lawful and urged Mr Hu to honour his "commitment to ensure the civil rights of citizens who peacefully express their views on public affairs".
They said that for the international community to take China's often-expressed commitment to human rights seriously, "it is urgent that China's central leadership ensure that no one be arrested or harassed simply for the peaceful expression of his or her views".
Brad Adams, Asia director for Human Rights Watch (HRW), which released the letter, said Mr Liu's detention was the most significant Chinese dissident case in a decade.
Mr Adams said the arrest indicated "a political hardening that runs against the current aspirations of the Chinese people".
The European Union and the United States have also appealed for Mr Liu's release.
A spokesman for China's Foreign Ministry told reporters that China would "solve the relevant issue within the legal framework.
"China firmly opposes any foreign interference in its internal affairs," said Qin Gang.