Chinese artist Ai Weiwei's reluctance to discuss his release fits a pattern indicating dissidents are being forced to buy their freedom with silence, in a setback for human rights, activists said.

Ai, who helped design Beijing's "Bird's Nest" Olympic stadium but has angered the government with his outspoken activism, was released Wednesday nearly three months after being detained in a crackdown on government critics.

He told AFP Thursday he was "happy to be free" and reunited with his family but ruled out further comment -- a far cry from his usual fearless criticism of a Communist Party government he has previously called "gangsters".

Yet Ai is just the latest in a string of normally forthright activists to suddenly go mum.

This indicates a new government strategy to silence dissenters, possibly through threats of further repercussions against them or even their families, said Phelim Kine, an Asia researcher with Human Rights Watch.

"We can only speculate on the types of threats and intimidation that may or may not have been employed to get such forceful and strong personalities to suddenly buckle," he told AFP.

Ai was among dozens of activists, rights lawyers and other government opponents rounded up since February as authorities moved to prevent any Chinese version of the "Arab Spring" protests that have raged this year.

Recent high-profile detainees who have been released have, with few exceptions, uncharacteristically refused comment on how they were treated and the conditions of their freedom.

These have included prominent rights attorneys Teng Biao, Jiang Tianyong, and Li Fangping -- vocal lawyers known for taking on sensitive cases often directed at government abuses.

Zeng Jinyan, the wife of jailed Chinese rights activist Hu Jia, told AFP earlier this month to expect the same, with Hu set to be released on Sunday after serving a prison term of three and a half years.

Police told her Hu was not likely to enjoy a "normal" life after his release, she said, indicating possible tight restrictions.

Shanghai human rights lawyer Li Tiantian, who was released in late May from three months' detention, wrote in a series of subsequent posts on Twitter that police presented her with intimate details of her sex life and threatened to ruin her reputation.

She later described her experience in the form of a fable posted on her blog, apparently to avoid angering authorities.

In the tale, an angry hornet attacks a bird with repeated stings to prevent the bird stirring up the insect's nest.

Eventually the bird is released but only after it promises "(1) not to speak of the past few months; (2) not to damage the hornet's reputation; and (3) not to urge other animals to stir up the hornet's nest."

Ai -- who reportedly confessed to tax evasion -- indicated Thursday that he would remain silent while facing the prospect of a possible trial for alleged economic crimes.

"I'm on bail so I can't give out any information," he told AFP.

China's foreign ministry on Thursday confirmed Ai's release on bail, but said he was still under investigation and could not leave his home "without permission".

Press freedom group Reporters Without Borders said in a statement that the threat of punitive action "prevents the victims from continuing their former activities and undermines them psychologically and financially."

"Obviously, they have concern for their freedom, for their careers, for the worries of their family members," Joseph Cheng, a political science professor at City University of Hong Kong, told AFP.

"It is quite likely that they will agree to keep quiet for a little while, come out and have a quiet life for a while, but it is a big loss to Chinese society."

The larger problem is that silencing people like Ai also muzzles, by extension, the people whose rights they advocate, Kine said.

One of Ai's main activist campaigns sought justice for parents of children killed in a devastating 2008 earthquake in southwestern China, during which widespread school collapses prompted accusations they were shoddily built.

"When even these small conduits are sealed off, it means the pressure has nowhere to go. That does not bode well for the stability of China," Kine said.