Xu has been detained dozens of times since 2009 for lobbying the government to grant education rights to migrant children in Beijing. That year, his non-profit legal aid centre Open Constitution Initiative was closed by the authorities and he was taken into custody for nearly a month.

When he was released, he was barred from resuming teaching at the Beijing University of Post and Telecommunications.

Yet none of the hardships have put him off.

"For the world to become a better place, someone has to pay a price," Xu said.

In May, he launched the New Citizen social movement, an initiative to push for democracy and basic civil rights. Participants are required to say no to corruption and injustice and pledge to live an honest life.

The authorities have since stepped up their surveillance of him. He was put under house arrest for a month in the lead-up to the Communist Party's 18th national congress last month. His everyday movements and communications are still closely monitored.

Last Saturday, days after he posted online an impassioned open letter to new party general secretary Xi Jinping , police held him for nearly 40 hours in what he said was a bid to stop him meeting supporters for dinner. Beijing's Public Security Bureau did not respond to a request for comment.

His letter attacked dictatorship, corruption and injustice under one-party rule.

"I hope you can show the courage and wisdom to lead China towards constitutional democracy," he wrote.

Scholars say the Communist Party is hostile to independent, organised groups with extensive networks which it fears could pose a challenge to its rule.

Xu insists the New Citizen movement is not a political party.

"We're above that," he said. "We're not aiming at toppling the regime … our role and mission is to build democracy and the rule of law."

His passionate determination may even remind the Communist Party of its early days, when a group of young intellectuals who deplored the corrupt Kuomintang government in the 1920s swore to fight dictatorship and pursue selfless and clean living.

"But we're different: our movement is an open, free and spontaneous citizens' movement," Xu said. "It's not an underground revolutionary party."

Xu said he and his supporters want to see the end of one-party rule and would support any party that was freely elected, "whether it's the Communist Party or Kuomintang".

Xu, who says he has Christian leanings, often stresses the importance of love in a country where the social contract has broken down and people's relationships with each other are filled with mistrust and animosity. "We need to change this atmosphere of hatred and hostility," he said. "I have a sense of mission that I believe comes from God, and … I hope there will be freedom, justice and love in our world."

Zhang Lifan , a historian, said Xu's determination, idealism and organisational ability would make the authorities feel insecure. "From its own experience, the Communist Party is extremely anxious about groups that grow large," he said.

But Xu said he is not afraid of jail. "I'll do what I should do … I think it's a glorious thing to sacrifice for the sake of social progress and fighting injustice."