Three Nobel Prize laureates as well as a string of scholars and human-rights activists are among those who have petitioned Chinese President Hu Jintao to release Liu Xiaobo, a leading dissident, the Wall Street Journal reported. In the letter, dated Monday, 150 people call on Hu to "honor your commitment to ensure the civil rights of citizens who peacefully express their views on public affairs." Nadine Gordimer, Seamus Heaney and Wole Soyinka were the Nobel Prize-winning signatories to the letter. According to human-rights groups, Liu was detained on December 8 for his involvement in an open letter issued by Chinese activists earlier this month calling for major political reform in the country. Beijing has not commented on the whereabouts of Liu, a former university professor who spent time in jail for his role in the 1989 Tiananmen Square protests.