A Chinese human rights activist was convicted of subverting state power and jailed for seven and a half years on Wednesday, a court said, the latest step in a sweeping crackdown on dissent.

Activist Hu Shigen was the second person to be put on trial as a result of last year's so-called "709 crackdown" -- named after July 9 -- when more than 200 people were detained, including lawyers who took on civil rights cases considered sensitive by China's ruling Communist party.

Hu pleaded guilty at the Second Intermediate People's Court in the northern city of Tianjin and said he would not appeal, the court said on a verified social media account.

It convicted him of a litany of actions including "spreading subversive thoughts", endangering national security and social stability, it added.

The trial took place under tight security, with scores of police and plainclothes personnel -- identifiable by small gold star pins -- stationed every few metres around the court.

Traffic was blocked off on the court side of the street about 300 metres to either side of the entrance.

Family members of those detained, particularly their wives, have complained of being constantly surveilled and denied access to the court proceedings.

The official news agency Xinhua, which described Hu as the leader of an underground church, cited him as admitting in court that he sought support for a "peaceful transition" away from Communist party rule.

"I wanted to discredit the administration of justice, the public security system, and the government," he said, according to Xinhua's report. "I wanted more people to agree with me, and incite people to mistrust the government.

"I instilled these ideas in other people with the purpose of achieving 'colour revolution'," Hu said, adding that he had "long been influenced by bourgeois liberalism", according to Xinhua.

He thanked police, prosecutors and the judges for "their help and education", the agency added.

AFP was not allowed access to the proceedings and only government-appointed defence lawyers were present.

A founder of the China Freedom and Democracy Party in 1991, Hu has been a prominent dissident and writer for decades.

He was released from prison in 2008 after spending 16 years in jail for trying to organise memorials for the Tiananmen Square killings.

Afterwards he became an elder at number of Beijing "home churches" -- considered illegal by authorities because they are not registered with the government and refuse to submit to official oversight -- said religious activist Xu Yonghai.

"He is a Christian -- a loving, responsible person," he told AFP.

Hu was detained in 2014 along with celebrated human rights lawyer Pu Zhiqiang after they attended a private seminar about the crackdown ahead of its June 4 anniversary, and he was held for several weeks.

- Tightening controls -

About a dozen lawyers and activists from the "709 crackdown" still remain under arrest on state subversion charges.

Activist Zhai Yanmin was found guilty of the same crime by the court Tuesday and handed a three-year suspended jail term after having "admitted" to prosecutors' accusations in court.

Chinese President Xi Jinping has overseen a tightening of controls on civil society since assuming power in 2012, closing avenues for legal activism which emerged in recent years.

Beijing law firm Fengrui, which has defended victims of sexual abuse, members of banned religious groups and dissident scholars, was at the centre of the crackdown.

Its director, Zhou Shifeng, is scheduled to stand trial on subversion charges this week.

Another prominent Fengrui attorney, Wang Yu, who was detained over a year ago, has been released on bail, a Hong Kong TV channel said Monday as it showed her praising her jailers.