Two days later, the Beijing Civil Affairs Bureau and police raided the office and took away documents, furniture, and all the computers and electronic equipment. Later, the civil affairs authorities said they would shut down the organisation.

But Dr Xu said the raid had no legal foundation and vowed the group's legal service and volunteer campaigns would not stop.

'I regard it as a slap in the face of all of civil society in China,' said Dr Xu, a law lecturer at Beijing Postal University and a prominent campaigner of many public rights cases.

'I feel that we're slowly but steadily walking on a long path, but suddenly some rubbish flew into our face. We wiped it off, cleaned ourselves up and kept walking. I have never borne grudges and I, together with my friends, will keep walking on this path for democracy, equality and the rule of law in this country.'

Dr Xu, 36, started on the path in 2003 when he, Teng Biao and Yu Jiang - all PhD students at Peking University Law School - petitioned the National People's Congress for personal rights to be guaranteed under the constitution. They also sought the protection of migrant workers in cities after a colleague, Sun Zhigang, died at a shelter in Guangzhou.

Their petition created a sensation among intellectuals and earned the three young men early fame. They were inspired to start the Open Constitution Initiative, or Gongmeng, with the goal of using their legal aid to help needy groups and eliminate inequality in the public sector.

Even though the organisation is a non-profit-making, non-governmental charity, it cannot register as an NGO because of the mainland's notoriously strict regulations. To become a recognised NGO, it first needs to secure government sponsorship. Without that, a civil rights group can only register as a private enterprise - meaning they are treated like profit-making companies and have neither tax exemption nor financial support.

'We have managed to take steps forward in a constructive and sensible way,' Dr Xu said. 'We always persuade people, especially petitioners, to be non-violent. We believe in the constitution, and we look for any possibility to co-operate with the authorities for positive changes in the system.'

And so the Open Constitution Initiative has grown to become the most pioneering civil rights NGO on the mainland. Last year, it collected from families information about melamine-tainted milk, much of which was not acknowledged by the government. Armed with the information, it helped the families seek higher compensation in the courts.

For many disadvantaged groups - whether they be landless farmers or tortured petitioners - the legal services provided by Dr Xu and his friends provide hope for justice in a country where the social, economic and legal cards are stacked against them.

They have also helped lawyers ask for direct elections at the Beijing branch of the All China Lawyers' Association, promoted the protection of lawyers' rights to meet clients in criminal and politically sensitive cases, and rescued petitioners from 'black jails' - the secret prisons kept by provincial governments in Beijing.

The group's reputation, however, has come at a cost for Dr Xu. He has been beaten several times by officials or guards while trying to rescue petitioners from black jails. He says police are pressuring the landlords of his office and his flat to evict him.

'Some pessimistic friends have guessed that the worst ending of all might be that I'll be put in jail.'

If jail is an injustice Dr Xu suffers, it will be another in a long series. Born in rural Henan in 1973, he said he became aware of injustice in the country at the age of 15 and 'made up my mind to bring change to society'.

Given his legal acumen, he could lead a much better life: he lives in a rented flat, and is single and poor. But to Dr Xu, it does not matter. 'I don't care what house I live in, what well-off life I should lead or what car I should drive. I never care. My happiness doesn't lie in stuff like that.

'I've always thought I needed to shoulder some responsibility for the prosperity of this country and the happiness of its people. I believe in clean, beautiful politics and want to dedicate myself to that.

'But one day, if this country can become a place where everybody leads an equal life, with happy smiles on their faces, with all the dignity and rights that human beings should have, I will acquire my ultimate happiness.'