The reformers argued that China's Propaganda department is now so powerful that it controls even the country's top leaders.

"Not only the average citizen, but even the most senior leaders of the Communist party have no freedom of speech or press," the letter said, before listing the occasions when Mr Wen's remarks about political reform had been censored inside China.

"On August 21, 2010, Premier Wen gave a speech in Shenzhen called, 'Only by pushing ahead with reforms can our nation have bright prospects'. [However] Xinhua News Agency's official news release omitted the content in Mr Wen's speech dealing with political reform," it said.

Again, on September 22 and 23, Mr Wen saw his comments on political reform on his trip to the United States dropped from news reports in China. A rare interview with CNN, in which Mr Wen pledged to fight for political reform despite "some opposition" from within his party, has also been blocked inside China.

"We would ask, what right does the Central Propaganda department have to muzzle the speech of the prime minister? What right does it have to rob the people of our country of their right to know what the prime minister has said?" asked the signatories. The letter, which was posted on Sina, one of China's most popular websites, was itself quickly deleted.

Li Rui, Chairman Mao's personal secretary and a former member of the Communist party's Central Committee, was one of the signatories calling for China to allow free speech and to abolish state control of the media.

Mr Li said that even an essay that he had written for the People's Daily, China's party newspaper in 1981, had been deleted from a recent book by censors. "What incredible folly it is that an old piece of writing from a Party newspaper cannot be included in a volume of collected works!" said the letter. "What kind of country is this? Such strangling of the people's freedom of expression is entirely illegal," said Mr Li.

The letter described the secretive operation of the Propaganda department. "If we endeavour to find those responsible [for the censorship] we are utterly incapable of putting our finger on a specific person. They are invisible black hands, often ordering by telephone that the works of such and such a person cannot be published, or that such and such an event cannot be reported in the media. The officials who make the call do not leave their names but you must heed their instructions."

Other signatories included Huang Jiwei, a former editor of the People's Daily, and Zhong Peizhang, a former senior official at the Propaganda department.