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A court in China has imposed a suspended three-year prison sentence on a human rights activist charged with subversion of state power, following a brief trial.

It was the first publicly acknowledged hearing in a year-long case shrouded in secrecy and involving hundreds of Chinese human rights activists.

The official Xinhua News Agency said a court in the northern city of Tianjin sentenced Zhai Yanmin, who was arrested in July last year as part of a countrywide government campaign which paralysed China's activist legal circles. Roughly 300 lawyers and activists were initially seized and questioned before most were released.

Zhai's is the first of four cases expected to be heard this week after prosecutors announced in July that they would try a lawyer, Zhou Shifeng, of Beijing law firm Fengrui - which worked extensively on human rights cases - and three activists who worked with the firm, including Zhai. More than a dozen others remain jailed, their legal status uncertain.

According to Xinhua, Zhai said in a court confession that the group of lawyers, citizens and petitioners who believed in "pushing the wall" - a Chinese expression for overthrowing the government - methodically hyped politically sensitive cases.

They organised popular rallies during controversial human rights cases to draw international attention and undermine the Chinese state, Xinhua quoted him as saying in remarks which echoed previous government accusations towards the group.

Xinhua said the court was told that Zhai and the three others had "conspired and plotted to subvert state power", and had "established a systematic ideology, method and steps to achieve it".

Lawyer Zhou's sentence was suspended for four years, meaning that although he will not go to prison, he will have to live under considerable restrictions and supervision. He also lost all political rights for the same period, making him ineligible to run for local councils or other offices.

The punishment was the most lenient allowed for those convicted of subversion, which carries a maximum sentence of life in prison. That may be a reflection of concern at the harm to China's international reputation among officials, as well as a relentless campaign by family members and human rights groups to draw attention to the cases.

Police cordoned off the Tianjin No 2 Intermediate People's Court on Tuesday, one day after protesters flanked by foreign diplomats demanded more information about the cases.

The trial was attended by five foreign media outlets invited by the court and other observers, according to Xinhua, in an apparent attempt to address vocal criticism from the activists' supporters about a near-total lack of transparency surrounding the cases.

Many wives have said they and their retained lawyers have been denied access to the jailed activists for more than a year, receiving only occasional updates by word of mouth, while some family members seeking information have been briefly detained themselves.

Zhai's wife, Liu Ermin, was taken into custody on Sunday night and returned late on Monday to her Beijing home, where she is kept effectively under house arrest, friends said.

It was not clear whether Zhai would immediately be allowed to return home. A lawyer and legal assistant released recently have been filmed apparently recanting their actions, but their whereabouts remain unknown.