Chinese authorities are mounting a crackdown after an extraordinary letter surfaced on a Chinese Government website calling for the resignation of President Xi Jinping.

Space to play or pause, M to mute, left and right arrows to seek, up and down arrows for volume. Listen Duration: 3 minutes 9 seconds 3 m 9 s Listen to Matthew Carney's report Download 5.8 MB

The open letter, signed by "loyal communist party members", has triggered a fierce response from Chinese authorities.

At least 20 people have so far been arrested and family members have being detained in a campaign of intimidation.

By Chinese standards, it is an explosive letter; openly calling for the President's resignation is unprecedented.

And the suggestion that the letter has come from within the Chinese Communist Party is totally unacceptable.

The letter emerged on the government-backed Wujie News website earlier this month, but it is not clear who wrote it or whether party officials were involved in the letter.

It accuses Mr Xi of being a dictator and also of major economic and diplomatic blunders.

Authorities have mounted an extensive hunt for the authors, arresting at least 14 people from Wujie News including its top editors. Their friends and associates say they have gone missing in recent days.

Dissidents' relatives detained

Wen Yunchao, a Chinese dissident based in the United States, has been caught up in the crackdown.

"The authorities sense two crisis out of this letter: the first one is Xi feels a crisis about his own control and power, and the other is crisis of growing dissent within government," Mr Wen said.

Mr Wen has been accused of spreading the letter online, and last week his mother, father and younger brother went missing from their homes in the southern Chinese province of Guangdong. Since then, he has not been able to contact them.

"This is inhumane and barbarous. They're like bandits carrying out kidnapping," he said.

"This open letter has no nothing to do with my parents and my brother. And I am not involved in writing or publishing it."

Mr Wen said Chinese authorities were trying to force a confession and extract information from him.

"They threatened to abolish my brother's job. They intended to make me admit that I was involved in this case. Of course I can't accept that."

Prominent writer Jia Jia has also gone missing. He was accused of being involved with the letter and detained by Chinese police at Beijing Airport on his way to Hong Kong on March 15. He was released last Friday.

Meanwhile, another overseas Chinese dissident Chang Ping has just had three family members detained in China.

Amnesty International's China researcher William Nee said this latest harassment has hit a new low.

"We're calling for the Government to immediately stop harassment or detaining anyone who is connected to this letter, especially people who are family members of people who have commented on the letter from overseas," Mr Nee said.

"This goes against international law and standards and makes a mockery that China is ruled according to the law."

Since Xi Jinping came became President three years ago, he has concentrated power in his own hands and led a wide-scale crackdown on dissent, including jailing journalists, human right activists and lawyers.

The full text of the letter in English: