Mr Xu, who was seized from his home by security officials at dawn on 29 July, is the co-founder of Gongmeng, or Open Constitution Initiative, a free legal aid clinic for people who cannot pay for their legal defence.

The group has publicised, including on its website, a series of high-profile cases. It has provided legal aid to victims of baby milk formula tainted with melamine in which at least five children died and 300,000 got sick. It issued a report criticising the handling of unrest across the Tibetan plateau in March of last year. It has also come to the defence of petitioners detained in “black jails.”

For many years now, China’s Communist-controlled bar association has been “advising” its members not to take up controversial cases such as these, especially when they involve the state or public agencies. Many lawyers who disregarded this “advice” lost their licence to practice.

Since the authorities refused to register Gongmeng as a non-profit organisation, Xu had to register it as a business. For this reason the authorities now accuse him of tax evasion for failing to pay 1.42 million yuan (US$ 195,000) in taxes. If he is convicted Xu could get up to seven years of prison. Meanwhile Gongmeng's Beijing office and website have been shut down.

Mr Xu is well known for taking up in 2003 the cause of Sun Zhigang, a graphic designer who died after being beaten in a detention centre in Guangzhou. The case helped to bring about the abolition of the harsh detention centres used to hold and expel rural-urban migrants and vagrants accused of not having the right documents.

With Xu being formally charged, the case is closer to a possible trial. But whether it goes or not to trial, what is clear is that Beijing wants to silence any possible critical voice ahead of the People’s Republic’s 60th anniversary on 1 October, an important day for mainland authorities.