THE US-based dissident group Alliance for a Democratic China plans to launch a signature campaign in Hongkong this week for support for the nomination of Democracy Wall Movement activist Wei Jingsheng for the 1994 Nobel peace prize. Wei, serving the 14th year of a 15-year sentence in a prison near Tangshan for allegedly plotting against the Communist Government, is expected to be released next March. Despite his long jail term, sources said Wei refused to admit any guilt in exchange for his freedom. In March, Hongkong television showed him on a one-day tour of the industrial city of Tangshan. The footage was not screened in China. Dissident sources said although Wei had been treated ''relatively well'' in prison, he had lost his teeth after several hunger strikes when he was incarcerated in the remote northwest. He was transferred to Tangshan after Western governments stepped up their pressure on China to improve its human rights record. A statement issued by the dissident group yesterday said Wei's family in China would be informed of the campaign, which was supported by other foreign human rights groups. It also claimed that Beijing student activists had posted pro-democracy posters near the People's Heroes Monument in Tiananmen Square on July 1. The posters called for a lifting of the ban on political parties and press freedom in China. The alliance said the posters were immediately removed by police guarding the square and no one was arrested.