A group of Communist Party veterans has written a daring open letter calling for the removal of China's top security official, in the latest sign of disunity ahead of a once-a-decade leadership transition.

Security chief Zhou Yongkang, one of China's top nine rulers, is viewed as a hardliner and is linked to Bo Xilai, the charismatic party leader whose downfall earlier this year triggered the nation's biggest political scandal in decades.

The letter, published on a number of overseas websites, is an audacious step in China, where open dissent or organised criticism of top leaders is usually severely punished.

Zhao Zhengrong, a retired anti-corruption official from the southwestern province of Yunnan, told AFP he and 15 other party members from the same province had sent the proposal advocating Zhou's removal to higher authorities.

Zhou is expected to step down from the nine-member Standing Committee, and as head of the party's Politics and Law Commission -- which oversees China's judiciary, prosecution and police -- at the party congress later this year.

But removing him earlier would send a signal to the people on which direction the nation was heading, the open letter said.

"We are demanding this because Zhou Yongkang directed the 'Chongqing model' and supported Bo Xilai. They are liars, they are of the same ilk," Zhao said. Two other signatories also confirmed the authenticity of the letter to AFP.

Zhao was referring to Bo's rule of the southwestern metropolis of Chongqing, which included an anti-mafia crackdown that many say involved widespread use of torture, and a leftist revival that saw residents sing Maoist songs.

During his five years in the position, Zhou, who oversees a budget this year of $111.6 billion -- higher than China's declared military budget -- has gained a reputation as a hardliner willing to use force to quell unrest.

He has overseen the quelling of riots in Tibet in 2008, and in Xinjiang in 2009, as well as a crackdown on democracy and rights activists.

Analysts say Bo's March removal as Chongqing head and his subsequent suspension in April from the party's 25-member Politburo are indicative of a major split between hardliners and reformers at the helm of the ruling party.

The scandal is a huge embarrassment for the government, which had been keen to project an image of unity as it gears up for the sensitive handover to a new leadership later this year.

The letter, which is dated May 4 and can be seen at http://canyu.org/n48532c6.aspx, accuses Zhou of "giving active support" to Bo.

"Facts that are coming to light now, prove the behaviour of Bo Xilai did not lead to ordinary mistakes, but to serious crimes," it said.

"People like Zhou Yongkang, not only took part in Bo Xilai's plan for the 'Chongqing model', but helped push it forward and gave it active support. He should not be allowed to escape his crimes."

Rights activists have accused Zhou of manipulating the judiciary and police to aid Bo's extra-legal crackdown on organised crime in Chongqing.

"Bo Xilai's attack on the mafia depended on the judicial system under the leadership of Zhou Yongkang, including the police, courts and the prosecution," retired Shandong University professor and social critic Sun Wenguang told AFP.

"The methods they were using to attack the mafia were destroying judicial independence and judicial fairness. This led to a lot of unjust trials and further spread terror throughout society."

Sun said Zhou was also likely consulted on the arrest of rights activists including Chen Guangcheng, who recently sought refuge in the US embassy, and the convictions of activists such as jailed 2010 Nobel Peace Prize winner Liu Xiaobo and Gao Zhisheng.

"When they were striking at the mafia, they were also hitting out at those who held opposing views," Sun said.

"The dissidents and democracy activists, those with opposing views, were being grouped together with real criminals."

On Tuesday, state media gave prominent coverage to a May 9 speech in which Zhou pledged allegiance to President Hu Jintao and the rule of law, in a possible attempt to quell rumours that he had already been stripped of his responsibilities.