BEIJING (Reuters) - A prominent Chinese human rights lawyer arrested this year has had to endure hours of interrogation and his health has suffered, his wife wrote in a letter released on Friday, calling for his immediate release.

Pu Zhiqiang, one of China's most outspoken dissidents, was arrested in June on charges of causing a disturbance and illegally accessing personal information in a case that drew international condemnation.

He has denied the accusations, which have since been upgraded to include inciting ethnic hatred and separatism, charges which could lead to life in jail.

In an open letter to President Xi Jinping, released by one of Pu's lawyers, Shang Baojun, his wife, Meng Qun, said Pu was suffering from a series of medical conditions, including diabetes, high blood pressure, coronary heart disease and prostate problems.

"Over the last three months in detention, he has been interrogated almost every day, for more than 10 hours at a time," Meng wrote.

"Even a young and vigorous healthy person would not be able to stand it," she added. "He has been subject to inhuman mental and physical torment."

Pu, 49, a free-speech lawyer, has represented many well-known dissidents, including artist Ai Weiwei and activists of the "New Citizens' Movement", a group that has called on Chinese leaders to make their wealth public.

The charges against Pu add to evidence that the case against him is politically motivated, his supporters say. They come amid what rights groups say is the most severe clampdown on human rights in decades.

A trial is seen as being a long way off as prosecutors last week sent Pu's case back to the police for further investigation.

He was first detained in May after he attended a meeting in a private home to commemorate the bloody suppression of pro-democracy protests in and around Tiananmen Square in 1989.

Meng said Pu had dedicated his life to helping the weak, that the charges against him were ridiculous and that he did not hate anyone and had no enemies.

"I believe in his innocence," she wrote. "Please be merciful and let my husband come home."

Police could not be reached for comment.

(Reporting by Ben Blanchard; Editing by Robert Birsel)