China has formally arrested on "subversion" charges at least seven human rights lawyers and colleagues held in secret for six months in a sweeping crackdown on legal activism, family and associates said Tuesday.

More than 130 attorneys and legal staff were summoned or taken away in July for questioning in what campaigners called the fiercest attempt in decades to silence activists attempting to redress injustices in China's tightly controlled courts.

Zhou Shifeng, the founder of Beijing's Fengrui law firm, which was at the centre of the crackdown, has now been accused of "state subversion", which carries a maximum sentence of life in jail, his colleague Liu Xiaoyuan said on a verified social media account Tuesday.

It was the first time relatives have learnt the whereabouts of the 16 lawyers and their staff, who have been held by police in undisclosed locations for months.

The charges make it highly likely that the detainees will be tried, and face potentially lengthy jail terms.

Chinese courts are tightly controlled by the ruling Communist party and have a conviction rate of more than 99.9 percent, with forced confessions often used as evidence.

A week after he was arrested, state media said Zhou -- who provided legal advice to victims of a 2008 poisoned baby milk scandal -- "confessed" to an unspecified crime.

Trainee lawyer Li Shuyun, 24, is accused of the same charge as Zhou, Liu added. Fengrui lawyer Wang Quanzhang is also held for the charge, his sister said in an Internet post.

Four other people are said to be accused of "incitement to state subversion", which carries a maximum sentence of 15 years.

They include 24-year-old legal assistant Zhao Wei, her husband and mother both told AFP, citing a police notice.

"I feel deep grief," added mother Zheng Ruixia.

Zhou, Zhao and Li are being held at a detention centre in the northern port city of Tianjin, the notices said.

Friends also posted notices on social media showing that lawyers Xie Yanyi, Xie Yang and Sui Muqing had been formally arrested on the same incitement charge.

- 'Dark night' -

Over the past decade a small group of a few hundred Chinese lawyers used the courts to seek redress -- sometimes successfully -- for what they considered egregious rights violations.

They include victims of forced demolitions, illegal "black jails", dissidents jailed for their writing, and others detained for practising their religious faith.

State media -- which in the past sometimes praised rights lawyers' efforts -- have called the attorneys a "criminal gang" who created public disorder by organising protests outside courthouses.

China's ruling Communist Party does not tolerate organised dissent, and has tightened controls on civil society under the leadership of President Xi Jinping.

Despite official harassment, dozens of Chinese lawyers have come to the defence of their colleagues.

"We are defenders of human rights and the law, and possess an unyielding conviction that the rule of law will ultimately triumph over dictatorship," a New Year statement attributed to 300 lawyers posted online said.

"The smog is thick and the night dark, but the sun will shine as the time comes," they added.