China detained a total of 20 people following the publication of a letter calling on President Xi Jinping to resign, the BBC has learned.
The letter was posted earlier in March on state-backed website Wujie News.
Although quickly deleted by the authorities, a cached version can still be found online.
One of those held, columnist Jia Jia, appears to have been released, but most of the others are still believed to be in detention.
In most countries, the contents of the letter would be run-of-the-mill political polemic.
