A prominent Chinese human rights lawyer was convicted of subversion and sentenced to seven years in prison Thursday, state media reported -- the third such trial this week as authorities crack down on dissent.

Lawyer Zhou Shifeng was director of the Beijing Fengrui law firm, which has defended victims of sexual abuse, members of banned religious groups and dissident scholars, among others.

It was at the centre of the "709 crackdown" -- named for the July 2015 date on which it began -- which saw more than 200 activists and lawyers detained for involvement in cases considered sensitive by China's ruling Communist party.

Zhou was convicted of "subverting state power" and jailed for seven years, the official Xinhua news agency cited the Second Intermediate People's Court in the northern city of Tianjin as saying.

He pleaded guilty and said he would not appeal, Xinhua added.

The proceedings were held under air-tight security, with scores of police and plainclothes personnel -- identifiable by small gold star pins --stationed for blocks around the court, clustered every few yards and filming those present on camcorders and phones.

Traffic on the court side of the street was entirely cut off about 300 metres on either side of the entrance, with journalists diverted from the site and then closely trailed.

Zhou is the third 709 detainee to go on trial in Tianjin this week.

Authorities insist the Tianjin trials are open, with the court saying that more than 40 people including politicians, legal scholars, and "civilian representatives from all walks of life", as well as mainland and overseas media outlets, were present at Zhou's proceedings.

But family members of those detained, particularly their wives, complain of being constantly surveilled and forcibly being kept away from the court or seeking further information.

The court said in a social media post that Zhou had twice stated in writing that he did not want his relatives to attend his trial.

It included a photo of what it said was a handwritten letter signed and fingerprinted by Zhou, reading: "Given that my family members are all farmers who are not very well-educated, having them come to the court to attend my hearing would be of no benefit to either me or them."

AFP was not given access to the court Thursday, and in previous cases only court-appointed defence lawyers have been present.

Zhou's trial followed those of activists Zhai Yanmin and Hu Shigen, who were both found guilty of subverting state power by the same court.

Zhai was on Tuesday handed a three-year suspended sentence - considered relatively lenient by the standards of Chinese dissident prosecutions - for crimes that included waving banners and shouting slogans.

Hu, a prominent dissident and writer who previously spent 16 years in prison for trying to organise memorials for victims of the 1989 Tiananmen Square killings, was on Wednesday given seven and a half years, with the court stating he had sought to foment "colour revolution".