A prominent Christian human rights lawyer in China has been released after being detained by authorities for nearly seven months.

Zhang Kai posted on social media on Wednesday to announce he had been freed and was at his home in Inner Mongolia. It is unclear whether there are any conditions to his release or whether Zhang's health is affected.

China Aid, a human rights and religious freedom charity which has worked alongside Zhang in the past, said relatives have confirmed the post which said: "I am thankful for all friends who were concerned about me during this time and who looked after and comforted my family members."

The Christian lawyer had represented over 100 churches that faced forced removal of their crosses in Zhejiang province on the east coast of China. He also gave advice on how churches could defend themselves and spoke up for pastors who faced charges over their right to free speech.

On 25 August 2015, shortly before he was due to meet the US ambassador for religious freedom, Zhang was arrested. He was sentenced to six months to "black jail", which means he was under constant surveillance at an unknown location.

After six months of his detention, Zhang was shown on state television appearing to give a confession. This prompted anger among activists as many suggested the confession was coerced.

Zhang was subsequently charged with "endangering state secrets" and "disturbing the public order".

Bob Fu, founder and president of China Aid and a friend of Zhang, described him as a "bold human rights lawyer".

"I am very pleased to hear this good news, although further details about the conditions of his release are still unknown," said Fu.

"I appeal to Chinese authorities to release other arbitrarily imprisoned religious leaders, human rights lawyers and defenders, such as those arrested in July of 2015, including, attorneys Li Heping and Wang Yu, church leader Hu Shigen, and pastors Li Guozhi (Yang Hua), Bao Guohua and Gu Yuese."

Mervyn Thomas, chief executive of religious freedom charity Christian Solidarity Worldwide, also cautiously welcomed the news of Zhang's release, and called on authorities to allow "his freedom without condition".

Thomas continued: "We further express our concern that several of Zhang Kai's fellow lawyers, as well as a number of Zhejiang pastors, remain in detention. We urge the Chinese government to protect the rights and safety of those who defend freedom of religion of belief and other human rights in China."