Two overseas dissidents said on Wednesday that Chinese police had released family members they claimed were detained as part of an official probe into a letter calling on President Xi Jinping to resign.

Germany-based writer Chang Ping (pictured) and New York-based activist Wen Yunchao said their relatives had been held in connection to the anonymous missive that appeared online earlier this month.

Chang told AFP that his father and two brothers had been released on bail, but their activities were still restricted.

Police said they were bailed on Tuesday.

Wen told AFP that his father, mother and brother had been released after being held in Guangzhou in southern Guangdong province.

The three were not charged with any crime and security officials accompanied them to tourist sites during their detention, he added.

"I think my family's release is related to Xi Jinping's visit to the US," he said, referring to the Chinese President's participation in a Washington summit this week.

Wen earlier claimed that his father warned before his detention that officials in Guangdong believed the exiled activist had "helped spread" the letter.

Germany-based journalist Chang Ping said at the weekend that police in the southwestern province of Sichuan detained his family to pressure him to retract an article he wrote for German media.

The article accused Chinese police of holding journalist Jia Jia in connection with the anonymous letter.

Jia's detention was condemned by rights groups. He was released on Friday.

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

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

- Multiple detentions -

Chinese authorities have detained several people in what appears to be a reaction to the letter, which was attributed to "Loyal Communist Party Members".

It appeared on Wujie News, a state-backed website on the opening day of a high-profile political conclave, before it was deleted.

Those held include at lest four staff members at Wujie, who went missing around two weeks ago, a source at the outlet told AFP.

The BBC reported last week that a total of 20 people had been held in the probe.

Police in Sichuan province said Chang's three relatives had been held for suspected arson, and released a letter by Chang's brother Zhang Wei to state media in which he denied being held for political reasons.

Zhang's letter called on Chang "not to make these kinds of false statements which are even attacks on relevant departments and staff," according to the state-run Sichuan Online website.

It added three were held for accidentally burning woodland while making traditional graveside offerings to relatives, the report added. AFP could not confirm the origins of the letter.

A woman surnamed Tang at the Forestry Police bureau of Xichong county, where Chang's family members were held, told AFP: "They have carried out bail procedures and all three left public security at 2pm yesterday."

The fire was still under investigation, she added.

Chang said in an email to AFP that his father's and brothers' bail release meant they were "still marked as criminals".

He added: "They exist under the control and threat of the police, and they still do not enjoy freedom of expression because of fear".