Five Nobel Peace Prize winners have launched a campaign to free last year's laureate, jailed Chinese dissident Liu Xiaobo, saying they feared the world will forget that he is still imprisoned.

Ahead of Saturday's ceremony for the 2011 Nobel Peace Prize, past laureates and human rights groups announced a committee in support of Liu, a writer who is serving an 11-year sentence after writing a manifesto for democratic change.

The committee -- which includes South African anti-apartheid icon Archbishop Desmond Tutu -- voiced fear that Beijing is silencing Liu's family and friends after first carrying out an "international wave of intimidation."

"The international community seems to have forgotten that a year after the award ceremony, Liu Xiaobo remains in prison in China and in harsh conditions," the committee said in its first statement issued Thursday.

"The committee calls on all those committed to freedom of thought and opinion to join the committee in its efforts to obtain the release of Liu Xiaobo," it said.

Other Nobel laureates in the effort are Iranian rights lawyer Shirin Ebadi, anti-landmine campaigner Jody Williams and Northern Ireland campaigners Mairead Maguire and Betty Williams.

Former Czech president Vaclav Havel is among the non-Nobel laureates on the committee.

Liu, a former professor, co-authored Charter 08, a bold petition calling for political reform in one-party Communist-ruled China. He was convicted of subversion and sentenced to 11 years in prison on Christmas Day 2009.

China responded furiously to the Nobel Peace Prize and cut off political dialogue with Norway, which administers the award. Relations remain tense a year later.

In the past year, rights groups say that Chinese authorities only allowed Liu to leave prison briefly when his father died and have put his wife, Liu Xia, under house arrest without charges.

Carl Gershman, the president of the US Congress-backed National Endowment for Democracy, said that Liu's Nobel prize "deepened the Chinese government's legitimacy crisis".

The ceremony in Oslo, where Liu was represented by an empty chair, served as a "powerful indictment of the regime," Gershman told a hearing this week on Liu of the Congressional-Executive Commission on China.

"With all its stirring symbolism, the Nobel ceremony represented the confirmation by the international community of the sentiments of a good part of Chinese society," he said.

Liu Xiaobo is the only detained Nobel Peace Prize winner. Myanmar last year freed peace laureate and democracy leader Aung San Suu Kyi after she spent nearly two decades under house arrest.

Amnesty International and Human Rights Watch issued separate appeals Thursday for China to free Liu, with both advocacy groups fearing that Beijing is becoming more intransigent over human rights.

Sophie Richardson, the China director for Human Rights Watch, said that foreign governments should make progress on specific cases "a real benchmark for engagement with China" to show the "price to be paid for violations."

China's leader-in-waiting, Vice President Xi Jinping, and other senior officials "deserve no red carpet welcomes until there is evidence the human rights environment in China is improving," Richardson said.

Richardson also urged pressure over the cases of Gao Zhisheng, a lawyer for disadvantaged groups who has been missing for a year and a half, and Chen Guangcheng, a blind activist who exposed abuses in China's one-child policy and is said to have suffered beatings under house arrest after serving a jail term.