Chinese police have warned prominent government critic Hu Jia that he will be detained again if he continues his activism and speaks to foreign media, human rights defenders said Monday.

Hu, one of China's leading rights campaigners, was released from prison in June after completing a more than three-year sentence for subversion.

Police told Hu on Friday that he would be placed under "administrative detention" if he violated "any of the terms of his deprivation of political rights", Hong Kong-based Chinese Human Rights Defenders said in a statement.

Administrative detention allows police to hold anybody without trial or charge for a certain period and is typically used against people considered threats to social order, often activists.

Despite official efforts to muzzle him, the 38-year-old has vowed to continue his activism and has been campaigning for the release of other dissidents.

Last month, Hu publicly criticised proposed changes to Chinese law that would make it legal to detain suspects for up to six months, without charge, in secret locations away from police stations and official prisons.

Although not officially legal, so-called "disappearances" have become a popular method of silencing dissidents in a crackdown that began in February, when calls for Arab-style protests began appearing on Chinese websites.

Hu has also joined a growing online campaign to free blind activist Chen Guangcheng, who has been under house arrest in eastern China since completing a jail term of more than four years in September 2010.

Hu has posted photos of himself on his Twitter account wearing black sunglasses and a white T-shirt with an image of Chen and the words "Free Chen Guangcheng".

Police also told Hu on Friday that he was barred from travelling to Shandong province to visit Chen nor was he allowed to visit Liu Xia, the wife of jailed dissident Liu Xiaobo, who won the Nobel Peace Price in 2010.

Rights groups say the government has placed Liu Xia under house arrest without charge since her husband won the award.

Hu told AFP last month that despite an official ban on him expressing "opinions publicly" after his release from prison, he was determined to speak out.

"I told the police clearly -- I won't be restricted on issues concerning citizens' rights and benefits as well as freedom," Hu said.

"In this country the government is the one that violates human rights, police officers are performing tasks that violate human rights... so I must express my resistance in public, express my condemnation."