A UNITED Nations body has told the Chinese Government to arrange for doctors to check all detainees in Chinese prisons and detention centres in a bid to lessen the incidence of torture and ill-treatment. The UN Committee against Torture, in a statement issued at the weekend, also called on China to publish figures for the numbers of people held in prisons and detention centres, believed by some observers to be as high as 16 million. The UN requests amount to implicit acceptance that torture still takes place in Chinese prisons. Despite strong questioning last week, the Chinese Ambassador to the UN in Geneva failed to give figures for the total population in detention, sticking only to China's claim that 1.2 million are in prison - a figure that almost certainly excludes those held without trial. The committee, which monitors observance of the UN Convention against Torture signed by China in 1988, made a number of suggestions to help China stop torture, but made its strongest statement in the demand that all detainees ''should be examined by doctors immediately after their arrest and regularly thereafter''. The demand is likely to embarrass China, which fielded an unusually large delegation, including representatives of the Public Security and Justice Ministries in an attempt to resist criticism of its torture record by non-governmental groups such as Amnesty International.