BEIJING — Dozens of China’s most prominent writers and scholars are calling for the release of a dissident who was arrested after co-authoring a bold manifesto urging civil rights and political reforms.

Liu Xiaobo, 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.

Liu is currently being held at a police detention center in Beijing. His arrest was the most high-profile since human rights activist Hu Jia was detained ahead of last year’s Beijing Olympics.

The stifling of Liu and others critical of China’s one-party rule reflects the leadership’s anxiety about possible social unrest amid the global economic downturn and ahead of celebrations marking the communist regime’s 60th anniversary on Oct. 1.

“We can’t just stay quiet,” Li Datong, a veteran state newspaper journalist who was forced from a top editing job for reporting on sensitive subjects, said in a telephone interview today. “We have to defend our freedom of speech and our right to criticize the government.”

He is one of more than 50 people to have signed a petition being circulated on the Internet. The list of intellectuals who have also thrown their support behind Liu includes philosophy scholar Xu Youyu and economist Mao Yushi, as well as authors and people from the media from across the country.

“Freeing him would show our country is still on the road to democracy and rule by law,” said Su Yutong, a writer and environmentalist who added her name to the petition. “It’s also a way for China to show its stature as a responsible international power.”

The petition calls for Liu’s release and “the proper implementation of all free speech rights provided for by the Constitution.”

“His stances were rational and constructive,” said the document, which has been sent to China’s legislature. “His having been imprisoned for expressing critical opinions shows us that any one of us faces the risk of being silenced.”

More than 150 writers and rights activists, including Salman Rushdie, Nadine Gordimer and Wole Soyinka, have signed an open letter to China’s President Hu Jintao urging Liu’s release. Leader of the U.S. House of Representatives, Democrat Nancy Pelosi, also wrote Hu last month asking that Liu and other Chinese “prisoners of conscience” be released.

Liu, 53, is a former university professor who spent 20 months in jail for joining the 1989 student-led protests in Tiananmen Square.

He was one of the chief architects of “Charter 08,” an unusually open call 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.

It also called for the abolition of the criminal code that allows people to be imprisoned for “incitement to subvert state power” — the crime Liu has been accused of committing.

Of the more than 300 lawyers, writers and artists who signed the charter when it was released last December, Liu is the only one who has been arrested, perhaps an effort by authorities to warn the others.