A Chinese activist who has spent nearly half his life in detention has been freed from prison, reports said Wednesday, in a rare move amid clampdowns on other dissidents.

Xu Wanping was released this week from the Yuzhou jail in the southwestern mega-city of Chongqing, according to the New York-based campaign group Human Rights in China and US-funded Radio Free Asia (RFA).

Xu, a former factory worker and member of the outlawed Chinese Democracy Party, which advocates for an end to one-party rule, was sentenced in 2005 to 12 years in prison for "inciting subversion of state power".

He was freed this week after authorities in Chongqing granted him a three-year reduction, he told RFA Wednesday, without specifying a reason.

Despite his release, Xu will be deprived of his political rights for four years -- essentially a ban on speaking to media as well as restrictions on his movements and contacts -- and his wife told RFA she remains concerned about his physical condition after so many years in jail.

Xu has served a total of 20 years in various forms of detention. He was first jailed for eight years in 1989 for taking part in pro-democracy protests in Chongqing, and after his release was sentenced to three years in a labour camp in 1998 for inciting laid-off workers to protest.

In 2005, he was jailed again when Chinese authorities swept up several long-time democracy activists shortly after anti-Japanese protests in major cities.

The detentions apparently reflected China's fear that dissidents could take advantage of the anti-Japan demonstrations to influence ordinary people to protest over other issues including corruption and lack of freedoms.

News of Xu's release comes as other outspoken critics of the ruling Communist Party have reportedly been placed under detention ahead of June's 25th anniversary of the Tiananmen Square crackdown, a date that remains highly sensitive in China.

Among them is Gao Yu, a prominent journalist whose political writings have landed her in jail in the past.

Gao had been planning to attend a private Tiananmen-related gathering this week but has not been seen since last Thursday, according to multiple reports.