China will have to answer the questions during two hearings in Geneva at the beginning of November.

The UN Committee against Torture excoriated China for the "continued pervasiveness" of torture "in the criminal justice system".

It demanded a list of all the people detained in connection with the riots in Tibet in March, including their current location and convictions. It also asked for an explanation as to why detainees were denied access to a doctor during their imprisonment both before and after trial.

"At least 30 persons were found guilty and sentenced less than six weeks after the events, please clarify the basis of these sentences," the committee asked.

It demanded full details about the number of people who were killed in the riots, which spread from Tibet into the neighbouring provinces of Gansu, Sichuan and Qinghai. Reports at the time suggested that over 100 people had died in clashes with security forces.

The list of questions, which is dated August 2008, emerged just after Wen Jiabao, the prime minister, made China's first address to the United Nations General Assembly in New York.

It also coincided with a Chinese "White Paper" on the "protection and development of Tibetan culture."

According to Xinhua, the state news agency, the white paper was designed to "give the international community a better understanding of the reality of the protection and development of Tibetan culture" and to refute charges of "the so-called cultural genocide" in the region.

The paper attacked the Dalai Lama "and his cohorts" in the West for conspiring to "force the Tibetan ethnic group and its culture to stagnate and remain in a state similar to the Middle Ages."

China has compiled five reports to the Committee against Torture. In the latest report, dated June 2007, it said that the "prohibition of torture has been a consistent position of the Chinese government" and listed a number of regulations that have been brought into place to guard human rights.

Its report claimed that the number of criminal charges brought because torture was used to extort confessions had fallen from 143 in 1999 to 53 in 2004. However, the Chinese report failed to provide any detail on the cases or to comment on specific incidents raised by the UN.

In its latest grilling, the Committee also asked why China had ignored its recommendation to abolish "the Re-Education through Labour system", which sees dissidents sentenced to hard labour. "The system ... aims at altering the personality of the detainee [and] reportedly causes severe physical and mental pain and suffering," it said.

It also lashed out at the use of "retrievers (jiefang renyuan)", a large number of unofficial thugs who violently round up dissidents and take them to the authorities.

The UN also said it had heard that "the State party has established detention facilities especially to detain petitioners coming to Beijing" and asked for a full clarification, as well as for China to reveal the whereabouts of a number of high-profile dissidents and their legal representatives. It alleged that many of the dissidents had been "forcibly disappeared".