China has released a scholar and human rights advocate detained for almost a year, a rights group said on Tuesday, though his lawyer continues to be held.

Police in Beijing detained Guo Yushan, who ran a private think-tank, last October and later charged him with "illegal business activity".

He was released on Monday along with fellow think-tank member He Zhengjun, the Hong Kong based advocacy group Rights Defence Network said.

China's ruling party under president Xi Jinping has tightened a crackdown on intellectuals, lawyers and activists critical of the government, with scores detained and dozens jailed.

The scholar's release comes ahead of a visit by Xi to the US later this month. China's detention of dissidents is often a source of contention between Washington and Beijing.

Guo is best known for aiding the 2012 escape from house arrest of blind Chinese lawyer and dissident Chen Guangcheng, who dramatically sought refuge in the US' Beijing embassy.

His involvement reportedly earned Guo several months of house arrest.

Guo founded the "Transition Institute" which held talks and conducted research advocating economic and social liberalism. It was shut down in 2013, and several associates detained.

Police held one of Guo's lawyers, Xia Lin, in November. He remains in detention along with several other prominent human rights attorneys.