China's campaign to stifle dissenters like artist Ai Weiwei through secret detentions could come cloaked in a thicker veneer of legality if proposed crime law changes go through, a prominent rights advocacy group said on Thursday.

China has clamped down hard on dozens of prominent critics and dissidents including human rights lawyers, bloggers and civil activists by locking them up incommunicado for weeks and even months at a time in undisclosed locations.

China's moves to amend criminal laws next year to effectively legitimise extra-judicial detentions of people for up to six months could see a hardening of the nation's security crackdown in the run-up to a change in leadership, Human Rights Watch said.

The government move to legalise practices which it has otherwise indicated it should eradicate we find highly, highly problematic, Sophie Richardson, China director of Human Rights Watch, told reporters.

We believe this is indicative of a much larger scope that's being given to the domestic security authorities that will in the future be extremely difficult to rein in.

China's Foreign Ministry said the competent authorities of China have been soliciting the public's views on the proposed amendment.

We are willing to listen to their views but some organisations have been viewing China with coloured lenses, ministry spokesman Hong Lei told reporters. For such organisations, we will not comment on their behaviour.

Despite international criticism, China has continued to run so-called black jails -- unlawful secret detention facilities used to hold critics and petitioners, where detainees are often subjected to beatings, sleep and food deprivation, as well as psychological abuse.

With President Hu Jintao and Premier Wen Jiabao due to step aside for a new generation of leaders next year, the jockeying for power will likely see few liberal gestures, given the obsessive focus on preserving stability.

There are very few people, particularly at senior levels of the government, that are in a mood to experiment or take risks or propose innovative new strategies for responding to dissent in a systematic way, said Richardson.

Despite this, Human Rights Watch said civil rights activism was proliferating quickly as individuals, empowered by the Internet and wildly popular micro-blogging sites, were increasingly emboldened to push back to assert their rights.

She cited the case of blind activist lawyer Chen Guangcheng, beaten by thugs whilst under house arrest in northeastern Shandong province, and Ai Weiwei, who have become rallying points amongst regular Chinese citizens railing against human rights abuses.

In the long run, it will be very difficult for the Chinese government to put that genie back in the bottle, she said.

Ai said this week he was agonising over whether to pay a 15 million yuan $2.4 million (1.5 million pounds) bill for alleged tax evasion and tacitly admit guilt, or to fight the charge and possibly risk detention again.

Supporters of Ai, whose 81-day secret detention earlier this year sparked an international outcry, have said the tax case is part of Beijing's efforts to muzzle dissent.

(Additional reporting by Sui-Lee Wee; Editing by Nick Macfie)