A prominent Chinese human rights lawyer has "confessed guilt" a week after being detained in a massive crackdown on legal activists, China's state-run media said.

Zhou Shifeng is one of more than 130 lawyers detained or called for questioning by Chinese state-security this month, according to tallies by rights groups.

The lawyer provided legal aid to families of children poisoned by milk powder from a powerful dairy firm, and this year defended an 81-year-old writer detained for criticising the ruling Communist party.

Zhou has reportedly not been able to meet with a family-appointed defence attorney. But the official Xinhua news agency said late Saturday that he "admitted guilt," and pleaded for a "second chance."

"Some things about my actions at the law firm were illegal... my mistakes were serious," the report cited Zhou as saying, apparently while in police custody.

Xinhua said that nine other lawyers connected to Zhou's law firm had been detained on criminal charges, along with several associates.

The detained lawyers "gave interviews to foreign media, spreading opinions attacking the party and the government, slandering the legal system and other such negative views," Xinhua added.

Those detained include female lawyer Wang Yu, known for defending poverty-stricken victims of forced demolition, sexual assault, illegal detention and other abuses.

In an apparent effort to put pressure on Wang's family, her 16-year-old son has been questioned several times this week by police in the city of Tianjin, a family friend told AFP.

Police are stationed outside Wang's parent's apartment in the city, and "follow the family whenever they go out," said the friend who asked not to be named for fear of reprisal.

China's courts have a near-100 percent conviction rate. State-run media said last year that police using torture to extract confessions was "not rare" in the country.

The ruling party says it hopes to promote "rule of law," and a growing number of lawyers over the past decade attempted to expose official abuses using the courts.

But President Xi Jinping has stressed the party's ultimate authority over the legal system, and limits on activism have tightened.

Chinese rights lawyers previously faced physical attacks, house arrest and prison sentences, but analysts see the latest crackdown as the heaviest yet.

More than 200 legal activists have been targeted by police since July 9, according to Britain-based Amnesty International.

Those detained include 130 lawyers, according to tallies by activists in China.

Zhou's family has appointed lawyer Yang Jinzhu to act as a defence attorney, but police have not allowed him to meet with his client, the Hong Kong-based advocacy group Rights Defense Network reported.

China's state media frequently reports "confessions," from criminal suspects who are still detained without access to lawyers, a practice decried as a violation of legal procedure.