Some lawyers, economists and religious and civil society leaders have expressed similar views but it is unusual for someone with Professor Yu's official standing to make such direct and detailed criticisms of core Communist Party policies. Professor Yu is known as an outspoken insider. As the director of social issues research at the Chinese Academy of Social Sciences' Institute of Rural Affairs he advises top leaders and conducts surveys on social unrest. He previously has warned of the rising cost of imposing ''rigid stability'' by force but has not previously been reported as speaking about such immediate dangers. ''Some in the so-called democracy movement regard Yu as an agent for the party, because he advises senior leaders on how to maintain their control,'' said Feng Chongyi, associate professor in China Studies at the University of Technology, Sydney. ''I believe Yu is an independent scholar. This speech is very significant because it is the first time Yu has directly confronted the Hu-Wen leadership [President Hu Jintao and Premier Wen Jiabao] and said their policies have failed and will not work.''

Pointedly, Professor Yu took aim at the policy substance behind two of Mr Hu's trademark phrases, ''bu zheteng'' [''stability'', or ''don't rock the boat''] and ''harmonious society''. His speech was delivered on December 26, the day after the rights activist Liu Xiaobo was sentenced to 11 years in jail for helping to draft a manifesto for constitutional and democratic government in China, called Charter '08. The sentence, which shocked liberal intellectuals and international observers, followed a tumultuous year during which the party tightened controls over almost all spheres of China's burgeoning civil society, including the internet, media, legal profession, non-government organisations and business. Professor Yu's speech has not been previously reported but has recently emerged on Chinese websites. He cited statistics showing the number of recorded incidents of ''mass unrest'' grew from 8709 in 1993 to more than 90,000 in each of the past three years.

''More and more evidence shows that the situation is getting more and more tense, more and more serious,'' Professor Yu said. He cited a growing range and severity of urban worker disputes and said Mafia groups were increasingly involved in state-sponsored thuggery while disgruntled peasants were directing blame at provincial and even central government. ''For seeking 'bu zheteng' we sacrifice reform and people's rights endowed by law … Such stability will definitely bring great social disaster,'' he said. Professor Yu's speech reflects deep disillusionment among liberal thinkers in China who had hoped Mr Hu and Mr Wen would implement political reforms. Dr Feng said he still hoped the two would ''do something'' to leave more than a ''dark stain'' on China's political development before stepping down in 2012.

''The conservative forces are currently very strong,'' he said. China's security-tightening and potential for future loosening were linked to a leadership succession struggle between Mr Hu and the Vice-Premier, Li Keqiang, on the one hand, and the former president, Jiang Zemin, and the current Vice-President, Xi Jinping, on the other. ''I haven't given up the hope that the Hu-Li camp may make some positive political changes to mobilise public support.'' . The latest edition of the newspaper Southern Weekend broke a two-decade taboo by publishing a photo of a youthful Mr Hu with his early mentor, former party chief Hu Yaobang, who was purged in 1987 for his liberal and reformist leanings. But Chinese internet search results for the names of both leaders were yesterday blocked for ''non-compliance with relevant laws''. A Beijing political watcher said such crackdowns were being led by officials who had the most to hide, which did not include Mr Hu or his allies. ''Corrupt officials have such a high and urgent interest in controlling the media and especially the internet,'' he said. ''The more they feel that their days are numbered due to the internet and free information, the more ferocious and corrupt they become, in a really vicious circle leading to final collapse.''