The mother of a prominent Chinese Christian human rights lawyer has published a harrowing open letter to her son who has disappeared since being arrested in August.

Zhang Kai has spent much of his career defending churches from ongoing persecution. He has been an outspoken critic of Chinese President Xi Jinping's human rights record and has offered legal advice to churches who face having their crosses removed by China's state crackdown on religious symbols.

However in August, Zhang was arrested by police while in a church in Wenzhou, Zhejiang province. He was charged with "endangering state security" and "disturbing the public order" to the outrage of human rights organisations and lobby groups.

Since his arrest Zhang has disappeared and there are reports that he is facing a severe prison sentence or the death penalty without having access to legal representation.

Yesterday Zhang's mother published an emotional letter to Zhang, mourning his disappearance and pleading for news of his whereabouts.

"Your lawyers and I don't have even the smallest bit of information about your situation now that you've been taken," she wrote in the letter published by ChinaAid.

"These days, I am terribly upset. There is nothing in my mind except missing you. I want to control myself and my state of mind, but your shadow constantly appears before my eyes."

Zhang's mother spoke of her son's achievements and his determination to seek justice. Despite excelling academically, Zhang resisted his mother's pleas to take on safe cases or to take a job with the government, the expected course of action for a high-flying Chinese lawyer.

"When you graduated from college, you passed the bar examination with the second highest result in the whole city," she wrote. "You succeed because you've studied well and have a solid foundation.

"Confronting the current domestic chaos, you fought against social injustice and hazardous challenges. Was it necessary? Must you give yourself more pressure? But you said: 'If a soldier knows he needs to defeat his enemy on the battlefield, will he not go? I must do this and I cannot fear the risks. I must take it upon myself to dare to do what is right'."

In her touching and powerful letter, Zhang's mother offers an insight into life under the Chinese government's regime. She wrote of her fear for her son and pleaded for his return.

"I am frightened, in dire straits, and extremely nervous about your situation.

"My friends and colleagues praise you as 'my good son.' I should feel proud, but instead I am consumed by worry and fear."

Zhang's mother, who was not named in the letter, ended with a note of optimism saying that she hoped the current government will quickly come an end. As for Zhang, his fate remains unknown.