BEIJING — More than 160 prominent writers, scholars and human rights advocates outside mainland China have signed an open letter to President Hu Jintao asking him to release a well-known intellectual and dissident who was detained this month. The letter was posted on the Internet on Tuesday.

The letter to Mr. Hu indicates that the case of the intellectual, Liu Xiaobo — one of the driving forces behind a bold manifesto demanding democratic reforms that has received worldwide attention — is quickly turning into the latest human rights cause célèbre in China. The call for his release could embarrass the Communist Party at a time when Chinese leaders are celebrating the 30th anniversary of the policy of “reform and opening up.”

Among the writers signing the letter are three Nobel laureates in literature — the South African novelist Nadine Gordimer, the Irish poet Seamus Heaney and the Nigerian novelist Wole Soyinka — as well as other writers who regularly champion freedom of expression, including Umberto Eco and Salman Rushdie.

Just as notable is the fact that foreign China scholars also signed the petition, possibly risking their access to the country. Academics specializing in Chinese studies are often cautious about taking stands on political issues deemed sensitive by the Communist Party because the Chinese government has a track record of denying visas to people who publicly oppose the party’s views. Some of the scholars who signed the petition are already on the Chinese government’s blacklist, but others still have regular access to the country.

The scholars include Geremie R. Barmé of Australian National University; Richard Baum of the University of California, Los Angeles; and Andrew J. Nathan of Columbia University.

Prominent scholars in Hong Kong, which is controlled by China but enjoys greater freedoms than the mainland, also signed the letter.

Mr. Liu, a 53-year-old literary critic who has directed the Independent Chinese PEN Center, a group of writers who advocate broaderfree speech, was taken by security officers from his home on the night of Dec. 8 and has not been heard from since.

Human rights advocates say Mr. Liu has been made a target because he was one of the driving forces behind Charter 08, the recent manifesto demanding democratic reforms and accountability from the Communist Party that was signed by more than 300 Chinese and posted on the Internet.

Others who signed the manifesto have also been detained and questioned. All except Mr. Liu have been released.

The officers who detained him took computers, cellphones and personal papers from his home. His wife and other family members have receivedno word of his whereabouts or condition.

The open letter to Mr. Hu says: “For the international community to take seriously China’s oft-stated commitment to respect human rights and the rule of law, and for China’s own citizens to trust the judicial system to redress legitimate grievances, it is urgent that China’s central leadership ensure that no one be arrested or harassed simply for the peaceful expression of his or herviews.”

The letter notes that although Mr. Liu has been detained in the past for several years, he has never been convicted of a crime.

Mr. Baum, the political scientist at the University of California, circulated the petition on Chinapol, a Listserv managed by Mr. Baum that is read by many scholars of China. In an interview via e-mail, Mr. Baum said that he usually tried to avoid using the Listserv for political causes but that this case was different.

“While I have always tried to maintain Chinapol’s political neutrality, some violations are so egregious that I cannot, as a sentient being, remain neutral,” he said in an e-mail message.

Bruce Jacobs, a professor of Asian languages and studies at Monash University in Australia, said he signed the petition because “Liu was clearly arrested because of Charter 08.”

Mr. Liu supported the Tiananmen Square protests of 1989 and continued his dissident writings afterward, work that led to his detention by the authorities. Starting in 1996, he spent three years doing hard labor for having “repeatedly stirred up trouble and disrupted public order.” Since 1999, he has been allowed to continue his activism, but has been under surveillance.

Nicholas Bequelin, a China researcher for Human Rights Watch, said that if Mr. Liu was formally arrested and charged, that would mean Chinese leaders wanted to show intellectuals that the Communist Party was hardening its line and was unwilling to tolerate any dissent.

“He’s been detained before,” Mr. Bequelin said. “But if they send him to jail, that sends a political signal.”