Chinese prosecutors on Friday said they had formally charged a prominent human rights lawyer and three activists with "subverting state power" a year after detaining them in a sweeping crackdown.

Attorney Zhou Shifeng and activists Hu Shigen, Gou Hongguo and Zhai Yanmin will almost certainly face trial, following the announcement by prosecutors in the northern city of Tianjin on their verified microblog.

The men have been held since last July when more than 200 lawyers and activists were detained in a swoop on those who had taken on civil rights cases considered sensitive by China's tightly controlled courts.

Around a dozen are still being held without access to independent lawyers or family members, accused by police of subverting state power.

The crackdown has sparked international condemnation, with the United Nation's human rights commissioner as well as the European Union, United States and human rights groups calling for the lawyers to be freed.

The law firm at the centre of the crackdown, Fengrui, had defended dissident intellectuals and sought to draw public support using social media and banner-waving protests outside court houses.

The state subversion charges against the men can result in a maximum sentence of life in prison.

Zhou provided legal aid to families of children poisoned by milk powder in a 2008 scandal and as an employee of Fengrui defended an 81-year-old writer detained for criticising the ruling Communist party in a case last year.

State media has branded the firm a "criminal gang" and accused its lawyers of disrupting court hearings.

The official Xinhua news agency cited police as saying last year that Zhai "organised multiple mass protests to influence court sentences".

Hu and Gou are both Christian activists associated with "underground" churches in northern China.

Under President Xi Jinping, China has tightened controls on civil society, and the 709 crackdown -- named after July 9, the day when a large number of activists were detained -- represents its largest-scale operation in years.

The wives and family members of the lawyers and activists last week wrote a public letter accusing state security of "harassment" on the anniversary of their loved ones going missing.

Zhai's wife Li Ermin told AFP that police officers beat her after she travelled to Tianjin to try to locate her spouse.

Gou's wife Fan Lili said her cash savings and bank cards had been confiscated after her husband was detained, and police pressured her landlord to force her and her newborn baby to move out of their flat.