BEIJING (REUTERS) - China has detained a key campaigner for officials to reveal their wealth, a close friend of the activist said on Wednesday, as the new government escalates a crackdown that underscores the limits of its fight on corruption.

The detention of Xu Zhiyong, who has pushed for greater civil rights, could lead to a trial and trigger a global outcry over Beijing's tightening grip of a fledgling movement for officials to disclose assets.

China has detained 16 activists in the disclosure campaign, in what rights groups say is the new leadership's first crackdown targeting graft campaigners.

"It's possible more people will gradually be caught," said Mr Teng Biao, a close friend of Xu, the best known of the activists and the founder of the "New Citizens' Movement".

Xu has been held under house arrest for three months, with no reason given by the authorities, Mr Teng, a former human rights lawyer, told Reuters by telephone, adding that there was a high possibility he could face trial.

Beijing police detained Xu late on Tuesday night on a charge of "suspicion of gathering a mob to disturb order in a public place", a copy of his detention notice obtained by Reuters says.

But the police declined to confirm the detention to Reuters.

Xu's mobile phone was turned off on Wednesday. His wife, Cui Zheng, declined to be interviewed.

The detention is likely to draw the attention of Western governments, which have sparred with China over human rights.

The news came as China prepares to put on trial three activists agitating for officials to disclose assets.

President Xi Jinping's appointment as Communist Party chief in a once-in-a-decade leadership change last November had inspired many Chinese with hope for political reform, spurring citizens nationwide to push for the asset disclosures.

But the detentions signal the Communist Party will not tolerate any open challenge to its rule, despite the claims of greater transparency.

"This crackdown not only flies in the face of Xi's rhetoric, it also undermines Xi's legitimacy," said Ms Maya Wang, a researcher at Human Rights Watch. "The right thing to do would be for the government to release the activists."

Xu has been a thorn in the government's side. In 2009, he was briefly arrested on tax evasion charges his defenders said were trumped up in a bid to stifle his work. The charges were dropped after a public furore.

In May, he told Reuters he was under house arrest. "It could be due to my campaign to push for asset disclosure," he wrote in an e-mail message.