The wife of a prominent Chinese human rights lawyer arrested in June has written an open letter to President Xi Jinping pleading for his release, voicing concern over the "inhuman torture" he faced in prison.

"Please have mercy and allow my husband to come home!" Pu Zhiqiang's wife, Meng Qun, wrote in the letter seen by AFP on Friday.

Pu, a prominent rights campaigner who has represented dissident artist Ai Weiwei, was arrested in June for "creating disturbances and illegally obtaining personal information", charges his legal team say are politically motivated.

Meng appealed to Xi on both legal and humanitarian grounds, citing guarantees of freedom of expression in the Chinese constitution and her husband's failing health.

One of Pu's lawyers, Si Weijiang, released the letter on China's popular instant messaging application WeChat.

"He is suffering from several diseases including diabetes, high blood pressure, hyperlipidemia and heart disease," Meng wrote in the letter, dated Thursday and also posted on social media.

"He was interrogated for almost 10 hours every day during the first three months in the detention centre... (and) he was subjected to inhumane torture both physically and mentally."

Meng wasn't immediately available for comment, but Si confirmed she wrote the letter.

"Pu Zhiqiang has received medical treatment in the detention centre, but it's not enough," Si told AFP.

Chinese authorities routinely round up outspoken critics of the Communist Party in the weeks before key events, and Pu was among several people detained after attending a private seminar commemorating the 25th anniversary of the Tiananmen Square crackdown.

More than 40 journalists, lawyers, scholars and activists were held under various forms of detention ahead of the June 4 anniversary, Amnesty International said, in a larger clampdown than in previous years.

Meanwhile a wider crackdown on dissent has been under way since Chinese President Xi Jinping took office last year.