Zhou Shifeng, of the Beijing-based Fengrui Law Firm, is the third legal rights advocate to be convicted in as many days as part of a four-day Communist Party show trial in the northern city of Tianjin.

As with the two men tried before him – Hu Shigen and Zhai Yanmin – Zhou, 51, was refused his own legal representation and assigned a court-appointed lawyer. His family was barred from the courtroom and the judges handed down their verdict and sentence within hours of the trial commencing on Thursday.

Zhou and his associates are the first group to be tried since a wide government crackdown that began last July led to more than 300 lawyers and activists being detained and interrogated. Of those, about 20 have been formally arrested and charged.

Fengrui became a central target as authorities grew increasingly intolerant of the law firm's activist stance and penchant for taking on the most politically sensitive of cases.

The firm took on one of the China's biggest dairy companies responsible for the country's infamous tainted baby formula scandal, and its lawyers have represented dissident artist Ai Weiwei, Uighur scholar Ilham Tohti, and various labour rights activists and members of the banned Falun Gong movement.

Most recently, state media outlets said Fengrui's advocacy for the victim of a contentious fatal police shooting was aimed at inspiring social unrest.

In keeping with official party rhetoric portraying those on trial as having conspired with foreign agents looking to subvert the Chinese state, court prosecutors on Thursday said Zhou had come under the prolonged influence of "anti-China forces", which gradually motivated him to "overturn the country's political system".

They said this involved selecting cases that highlighted social injustices or were otherwise politically sensitive, organising protests in public and posting subversive messages on social media. Zhou pleaded guilty to the subversion charges and said he would not appeal, according to state-run news agency Xinhua.

One of Zhou's last clients was Zhang Miao, a Chinese news assistant for German newspaper Die Zeit who was detained for nine months after Chinese authorities took issue with the newspaper's coverage of Hong Kong's pro-democracy protests in 2014.

"It is confession obtained via torture," said Zhang, referring to the guilty pleas of the men tried, and the public "confession" delivered by high-profile lawyer Wang Yu.

Zhang said she was subject to sleep deprivation and intense psychological pressure while being interrogated for days on end. "If they were [physically] free, and free to express their views, I don't think it will be the current situation."

State media outlets have reported that Zhou and Zhai both requested that their families not be present at their trials. But wives and other family members, who have been unable to contact the men since their arrests, dispute this and say they are being closely monitored by state security and barred from entering the Tianjin courthouse.

With Sanghee Liu