Although quickly deleted by the authorities, a cached version can still be found online.
In most countries the contents of the letter would be run-of-the-mill political polemic.
"Dear Comrade Xi Jinping, we are loyal Communist Party members," it begins, and then cuts to the chase.
"We write this letter asking you to resign from all party and state leadership positions."
But in China, of course, and in particular on a website with official links, this kind of thing is unheard of and there have already been signs of a stern response by the authorities.
`Gathering of all power`
The detention of a prominent columnist, Jia Jia, was widely reported to be in connection with the letter.
Friends say he simply called the editor of Wujie to enquire about it after seeing it on line.
But now the BBC has spoken to a staff member at Wujie who has asked to remain anonymous and who has told us that in addition to Jia Jia another 16 people have been "taken away".
The source said they included six colleagues who work directly for the website, including a senior manager and a senior editor, and another 10 people who work for a related technology company.
And a well-know Chinese dissident living in the US said three members of his family, living in China`s Guangdong Province, had also been detained in connection with the letter.
Wen Yunchao said he believed his parents and his brother had been detained because authorities were trying to pressure him to reveal information. But he told the BBC that he knew nothing about the letter.
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