Release International, which supports persecuted Christians around the world, has launched a petition calling on China's president Xi Jinping to release the jailed Christian human rights lawyer Zhang Kai.

Zhang has been a prominent figure in the defence of Christians targeted for opposing the removal of crosses and the demolition of churches in Zhejiang province. He was arrested and appeared on television making an evidently scripted 'confession' in which he admits endangering national security and confessing "deep remorse".

He is one of more than 200 Chinese lawyers who have been held illegally and denied legal representation. Other prominent Christian leaders who opposed the cross removals have been arrested.

"Release is concerned this campaign of intolerance in Zhejiang could gather momentum and spread across China," said chief executive Paul Robinson.

He added: "China is rounding up its human rights lawyers for attempting to uphold the rule of law and is denying them legal rights. What message does this cynical mockery of justice give to the rest of the world? Why is this emerging superpower so insecure that it has to resort to silencing its lawyers?"

China Aid president Bob Fu, a former Tiananmen Square protester, said: "Zhang looked like he was under duress while making those scripted remarks about his confessed crimes of 'endangering national security' and 'gathering a mob to disturb social order'... he must have been going through enormous suffering and torture in the past six months."

One observer likened him to a concentration camp prisoner. "There's no way those were his own words," pastor Zhan Gang, deputy director of the Protestant Chinese House Church Alliance, told reporters.

"Lawyers like Zhang Kai, who love their country, and simply want to use the law of the land to contest the legality of church demolitions are being treated like enemies of the state," Robinson said.

Release International is encouraging supporters to sign a petition for his release addressed to President Xi Jinping. It says: "We respectfully call upon the Government of the People's Republic of China for the unconditional release with immediate effect of Zhang Kai, along with hundreds more detained simply for their defence of human rights and freedom of religion or belief."