Beijing is wary of the power of the internet

Liu Di had been held without charge since November 2002 after she allegedly criticised the jailing of a prominent internet dissident, Huang Qi.

Ms Liu - who wrote as Stainless Steel Mouse - and two other cyber-dissidents, Li Yibin and Wu Yiran, were freed on Friday, sources in Hong Kong said.

President Hu Jintao is said to have expressed concern about their cases.

Details of the releases were given by the Hong Kong-based Information Centre for Human Rights and Democracy.

The Centre said Liu Di had been granted bail, which is a rare move with Chinese dissidents, so her release might indicate she is free to return to her post-graduate work in psychology at a Beijing university.

Correspondents say the freeing of the three internet dissidents could be linked to the forthcoming visit of the Prime Minister, Wen Jiabao, to the United States, which is highly critical of Beijing's human rights record.

Web surveillance

China's authorities have been keen to promote the commercial potential of the internet, but are anxious to control its political content.

The campaign group Reporters Without Borders estimates that China employs 30,000 people to watch what its people are doing online.

Filters installed by the government ban access to foreign websites run by dissidents, human rights groups and some news organisations.

The content of domestic sites is also carefully monitored.