BEIJING - Dozens of China's most prominent liberal intellectuals have issued a petition urging the release of Liu Xiaobo, the veteran dissident facing subversion charges after promoting a petition demanding an end to one-party rule.

Liu was formally arrested on Tuesday, charged with 'inciting subversion of state power', following his detention late last year for promoting the 'Charter 08' petition, which challenged the underpinnings of Communist Party rule.

The move brings one of the most prominent critics of the Party a step closer to trial, and his case has galvanised criticism from human rights groups at a time when China's Internet controls also face controversy.

Now some of China's best-known academics and writers have added their voice to the outcry.

'We urge the immediate release of Dr Liu Xiaobo, and the true exercise of freedom of expression as promised by the Constitution,' stated the petition, dated June 24 and received by Reuters on Friday.

'Liu Xiaobo's troubles show the environment each of us must survive in ... He cannot freely express his views, and that means that none of us can freely and openly express our views.'

The 53-year-old dissident has a Ph.D. in Chinese literature and was active during the 1989 pro-democracy protests quelled by the bloody June 4 crackdown.

Late last year, he was among 303 dissidents and rights activists who launched 'Charter 08', a petition calling for the dismantling of one-party rule and creation of multi-party democracy.

The 52 signatories of the new petition include Xu Youyu, a philosopher; Mao Yushi, a prominent liberal economist; and Qin Hui, an historian whose writings on rural China and on economic reform have been widely influential.

Xu told Reuters that the petition, addressed to the country's Party-controlled parliament, showed that Liu's case will remain a lightning rod for opposition to political controls.

'This is intended to send the government a signal that intellectuals and scholars with some influence are very worried about Liu,' Xu said by telephone.

'His case is a blow to freedom of expression.'