In a demonstration of the increasing lengths Mr Xi is taking to crush dissent, the mainland-based relatives of two prominent overseas Communist Party critics have been detained in connection to the broadening investigation this past week.

Chang Ping, a well-known Chinese investigative journalist now based in Germany as a commentator, said his two younger brothers and a younger sister were taken by police in China's western Sichuan province on Monday.

Wen Yunchao, an activist living in New York, said his parents and younger brother in southern China had been missing since Tuesday. Mr Wen said Chinese authorities suspect him of helping to disseminate the letter, and have abducted his family as leverage for him to reveal information on the letter's source.

"But I can't confess to something that has nothing to do with me," he told Fairfax Media.

It remains unclear who authored the open letter, which surfaced first in early March on overseas-Chinese website Canyu on the eve of China's annual National People's Congress.

The letter, signed "Loyal Communist Party Members", also briefly appeared on the website of state-controlled Wujie News, before being swiftly taken down.

It is understood at least four editors and managers of Wujie have been detained since, as well as at least 10 staff at a related company providing technical support. Wujie's website and social media accounts have not been updated since mid-March and Wujie employees have told Fairfax Media they fear the company, with some 100 staff, may soon be shut down.

Chang said his family had been subject to threats and harassment after he published a commentary on German news channel Deutsche Welle's Chinese website criticising the disappearance of journalist Jia Jia on March 19, also in connection to the open letter.

Jia, who disappeared while preparing to board a plane from Beijing to Hong Kong, was released on Friday after 10 days in detention.

"Every citizen has the freedom of speech to engage in comment or criticism of the political activities of state leaders," Chang said in a statement.

He said his family had no understanding of his political beliefs or media work he engaged in: "I'd be in support of them, should they wish to cut off all ties with me at any point."

Jia's friends believe he was detained simply for calling an editor friend at Wujie, asking whether he was aware that such a sensitive letter had been published on the news website.

"For him to have disappeared for no reason, and then [be] released for no reason, this is not normal behaviour for a country with rule of law," Jia's lawyer Yan Xin said.

The open letter has attracted intense interest from watchers of elite Chinese politics for the way it eloquently criticises Mr Xi for excessively centralising power, an anti-corruption drive that has crippled the bureaucracy and poor handling of China's economic slowdown.

China's strict internet censorship has largely limited the letter's spread, and analysts have cast doubt on whether the letter originated from disenfranchised party members within China.

Political analyst Willy Lam said the government's heavy-handed crackdown highlighted the level of paranoia within the Xi administration that the letter's content could resonate if allowed to spread to a wider audience within mainland China.

"The sentiments do overlap with what many people who are critical of Xi Jinping think," the Chinese University of Hong Kong academic said. "If you add the people affected by the anti-corruption campaign and also relatively liberal cadres who are unhappy about the personality cult being built around Xi Jinping, then it's quite a substantial number of party cadres."