The wife of a prominent Chinese human rights activist says he was released Sunday after spending more than three years in prison for subversion.

Zeng Jinyan said in a Twitter message that Hu is safe and very happy. Hu's three and-a-half year prison sentence was set to end Sunday.

Hu was jailed in 2008, just months before the Beijing Olympic Games, after he used the Internet and interviews with reporters to highlight rights abuses in China.

Hu's release comes just days after prominent Chinese artist Ai Weiwei was allowed to return home following nearly three months in police detention.

Colleagues of Ai said Saturday that four of the artist's associates were also freed from detention this week.

The terms of Ai's bail prevent him from leaving Beijing or talking to the media.

Ai's arrest was part of a wider crackdown on Chinese government critics that began in mid-February.

China's official Xinhua news agency said Ai was released because he had confessed to tax evasion and promised to repay what he owes. It also said the diabetic artist was released on medical grounds.

Ai is one of China's best-known artists and an international celebrity. He has exhibited in the world's top art museums and had a hand in designing Beijing's “Bird's Nest” Olympic stadium.

His detention had become an issue in many of China's diplomatic encounters with other countries. The release came ahead of an important trip to Europe by Premier Wen Jiabao and a high-level U.S.-Chinese diplomatic meeting that took place Saturday in Hawaii.