Chinese authorities have detained the relatives of several dissidents living abroad, they said, as part of a widening crackdown following the publication of a letter critical of President Xi Jinping.

Germany-based journalist Chang Ping said local authorities in the southwestern province of Sichuan had detained his two younger brothers and a younger sister in connection with suspicions that he had been involved in writing an anonymous letter calling on Xi to step down for the good of the country.

"Numerous relatives in China have been subject to investigation, harassment, and threats" after he discussed the letter in an article and interview, Chang said in a statement posted on the web site chinachange.org.

Police had asked them to demand that he cease publishing any criticisms of the Chinese Communist Party, "or the government would find ways to charge my family members", he said in the document, dated Sunday.

Chang, a prominent commentator on contemporary affairs, was formerly a senior journalist at the outspoken Southern Weekend newspaper but moved to Germany after coming under sustained pressure for advocating more government opennness and accountability.

The detention of Chang's family is the latest example of what appears to be a widening campaign of intimidation aimed at people thought to be associated with the letter criticising Xi.

Last week, New York-based Wen Yunchao said officials in the southern province of Guangdong had taken away three of his family members.

Chang and Wen have both denied any connection to the letter.

Four members of staff at Wujie News, a state-backed website which carried the letter earlier this month before deleting it, have been missing since last week, a reporter at the outlet earlier told AFP.

According to reports 10 associated technical personnel have also been held.

Media criticism of top leaders is almost unheard of in China, where the press is strictly controlled by the ruling Communist Party.

The letter, seen by AFP in a cached form, berated Xi for centralising authority, mishandling the economy and tightening ideological controls.

"Due to your gathering of all power into your own hands... we are now facing unprecedented problems and crises in politics, economics, ideology, and culture," it said.

Chinese journalist Jia Jia was detained at Beijing's main airport ahead of a flight to Hong Kong last week, before being released Friday evening.

Rights groups linked his detention to an official probe into the letter.