The letter to Hu supporting Liu Xiaobo comes as the Chinese government celebrates the 30th anniversary of the country's turn to an open economy, The New York Times reports. The letter was posted on the Internet Tuesday.
Liu, director of the Independent Chinese PEN Center, was arrested Dec. 8. His family has not heard from him since he was detained.
Supporters believe he was picked up because he was one of the backers of Charter 08, a manifesto posted on the Internet urging more freedom and accountability in China.
Those signing the letter to Hu included novelists Wole Soyinka of Nigeria and Nadine Gordimer of South Africa and Seamus Heany, the Irish poet, all Nobel laureates. Salmon Rushdie, the Pakistani-born novelist who was the subject of a clerical fatwa, and Italian novelist Umberto Eco also signed.