It is thought that Zhang Kai was seized after defending churches against cross removal in China.
The government has cited health and safety reasons for the removal of the crosses.
Release International, a Christian charity, is calling for the release of the lawyer and says it is concerned that he could be tortured into confessing.
Zhang Kai was based in Beijing and had been defending churches in the courts against moves to tear down their crosses. It has been reported that officials in Wenzhou, in the east of the country, are now charging him with spying.
prominent HR lawyer Zhang Kai was taken away by PSB with 2 assistants from Wenling Church,Wenzhou at midnight 8/25/15 pic.twitter.com/H7NxYxyFBK
— Bob Fu??? (@BobFu4China) August 26, 2015 According to China Aid, he has been accused of "gathering a mob to disturb public order and stealing, collecting, purchasing and illegally providing state secrets and intelligence to overseas organisations."
Paul Robinson, Chief Executive of UK-based Release International, said: "It's deeply worrying to see a nation such as China rounding up its lawyers, and claiming it is doing so to uphold the rule of law.
"Release is adding its voice to the growing numbers calling for the immediate release of Zhang Kai.
"As a growing world power, China should embrace, rather than oppose, those who stand up for justice and righteousness under the law."
Zhejiang province is a heavily Christian area and it is thought that the authorities have removed around 1,200 crosses in the region.
Protestors to the demolition say it is a move by the government to slow the growth of the Christian faith in China.
It is estimated that there are between 67 and 100 million Christians in China; there are thought to be 86 million members of the Communist party in comparison.
Paul Robinson, from Release International, said some estimates show that there could be more Christians in China than members of the Communist Party: "It looks very much like a fear of losing control.
"The authorities appear afraid that Chinese Christians could have divided loyalties and that human rights campaigners who mount legal challenges in the courts are undermining the authority of the Communist Party.
"Neither is the case."
It is estimated that since July around 230 human rights lawyers and activists have been detained.
Zhang Kai was arrested at the end of August, ahead of a meeting with the US ambassador about religious freedom.
He was seized in Wenzhou where he was helping church leaders legally contest orders to tear down crosses from their churches.
He was charged with 'endangering national security' and 'gathering crowds to disturb the social order' and Release International is worried that this could develop into spying charges - which could carry the death penalty.
The charity has been denied any access to him and information about his whereabouts.
It is thought that he will be held for around six months in a secret location, known as a "black jail".
Human rights lawyers who have been interrogated in black jails have later said they were tortured while in custody.
Bob Fu, from China Aid has condemned the arrest: "The enforced disappearance of renowned religious freedom defence lawyer Zhang Kai and his arbitrary detention in jail for six months is absolutely outrageous.
"He did nothing but legally defend [against] hundreds of churches' crosses from being demolished."
Fu has called on President Obama to condemn China's "total disregard for basic human rights and religious freedom."
The roundup of lawyers comes ahead of a visit to the United States by China's President Xi Jinping next month.
Concerns about the impact of China's economy and potential unrest may also be behind political insecurity in the country.
Release Chief Executive Paul Robinson said: "Please pray for Zhang Kai and other courageous Christian human rights lawyers who are being arrested, detained and harassed.
"Pray they will know God's presence with them.
"Pray that they will be strengthened in their calling and know that their family around the world is standing with them."
