BEIJING (Reuters) - A popular website for Chinese Muslims has been inaccessible since Saturday after a critical letter addressed to Chinese President Xi Jinping was posted in one of its forums.

A Muslim man from the Chinese Hui minority reads the Koran as others listen in a sitting room in Aksu, Xinjiang Uighur Autonomous Region July 27, 2012. Picture taken July 27, 2012. REUTERS/Stringer

Users of China Muslim Net, one of China’s main websites carrying material by and about the Hui, a large and relatively well-assimilated community of China’s Muslims, say they have been unable to access the website since Saturday.

A critical open letter addressed to Xi had been uploaded in a discussion forum on the site hours before it became inaccessible.

Young, internet-savvy Hui consider the website to be an important forum for discussing matters relating to their religious practice.

Chinese officials recently said that religious extremism had begun to spread to central China from the violence-prone far western region of Xinjiang, mostly populated by ethnic Uighurs, who make up another large portion of China’s Muslims. [nL4N1DT2XS]

China’s constitution guarantees religious freedom, but rights groups say the officially atheist ruling Communist Party seeks to restrict religious practice, especially for Muslims. The Chinese government strongly denies such charges.

The reason for the website’s inaccessibility was not clear.

The internet address for China Muslim Net on Wednesday showed a message saying the website was under maintenance.

The Cyberspace Administration of China, the government’s internet regulator, did not reply to requests for comment on Wednesday. Messages and calls to the website’s chief executive went unanswered.

A copy of the critical letter posted on the site, and seen by Reuters, called for the release of a student named Kwong Pyong who the letter said has been unreachable since October, shortly after he shared photos online of a satirical T-shirt with a message likening President Xi to Hitler.

The student said he planned to wear the shirt in public.

Shortly after the letter was posted, a scholar at the Chinese Academy of Social Sciences, Xi Wuyi, shared a screenshot of it along with other China Muslim Net content on her Weibo account, saying that the website promoted religious extremism.

Previous Weibo posts by the same scholar have lambasted the website for its critical stance on atheism.

President Xi in July urged Chinese Muslims to resist illegal religious “infiltration”. [nL4N1A71X1]

Gu Yi, one of the letter’s three co-authors and a student activist studying in the United States, said he suspected the loss of access to China Muslim Net was related to the letter.

“Why would they target the website at this time?” he told Reuters on Wednesday. “Wuyi’s screenshots of the post of the open letter gained almost 500 reposts in several hours, making the open letter spread beyond their control.”