A group of 50 Vietnamese activists and rights organisations have written an open letter to Facebook chief Mark Zuckerberg suggesting his company may be colluding with communist authorities to scrub out online dissent.
Vietnam ranks among Facebook's top 10 users by numbers and the site is hugely popular among dissidents in the one-party state where independent media is banned and blog sites are routinely removed.
The letter to the head of the world's largest social media platform was signed by 50 organisations, activists and bloggers who said they have seen an uptick in "account suspensions and content takedown" since last year.
Vietnam's government said in April 2017 that Facebook has agreed to remove "bad and malicious" content that violates local laws, including fake news and imposter accounts, but made no explicit mention of anti-regime material.
"It would appear that after this high-profile agreement to coordinate with a government that is known for suppressing expression online and jailing activists, the problem of account suspension and content takedown has only grown more acute," said the letter published late yesterday.
"We urge you to reconsider your company's aggressive practices that could silence human rights activists and citizen journalists in Vietnam," it added.
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The group said that several Facebook posts were censored last week during a high-profile trial of six democracy activists who were handed heavy sentences for "attempting to overthrow the state".
Vietnamese officials and Facebook did not immediately reply to requests for comment today.
Internet in the country is classified as "not free", according to web watchdog Freedom House, the worst in Southeast Asia and second only to China in all of Asia.
Activist Le Van Son, who signed the letter, told AFP his Facebook page is frequently censored and his profile was taken down temporarily last week after he posted in support of the activists on trial.
"My Facebook account reflects my critical opinions and enables my right to talk about democracy, press freedom and freedom of expression in Vietnam," said Son.
"I have never violated regulations by posting racy pictures, false information or humiliating others with curse words." Vietnam announced last year a 10,000-strong military cyber force tasked with fighting "wrongful views" online.
Activists have said the online brigade, dubbed 'Force 47', has flooded their sites with pro-government commentary and harassment.
Unlike in China, Vietnam does not employ a Firewall to block major social media sites, although Facebook access is sometimes interrupted during protests in the country.
The letter to Zuckerberg comes amid deepening controversy at Facebook over privacy and security lapses after the revelation that British firm Cambridge Analytica -- which worked with Donald Trump's campaign -- hijacked data on millions of users.
Vietnam was among 10 countries affected by the breach, according to a Facebook blog post last week saying that data from more than 420,000 users in Vietnam may have been improperly shared with Cambridge Analytica.