The letter claims Facebook inappropriately suspended accounts and removed content belonging to human rights activists and journalists at the behest of the government.
It comes amid a rising tide of criticism of Facebook by groups in countries such as Myanmar and Sri Lanka over the platform's content moderation policies, which critics say have been too lenient with accounts that seek to spread violence and attack minority groups.
Facebook has also been accused of cooperating with repressive governments. In Cambodia, government officials have told BuzzFeed News they had a direct line to Facebook, which they use to suggest accounts and posts to take down, including those deemed to be too critical of the prime minister.
Zuckerberg is set to testify before members of two US Senate committees on “privacy, safety, and democracy” after a major data privacy scandal that affected up to 87 million Facebook users.
"We’ve noticed a troubling increase in the number of activist Facebook pages taken down and content removed. We have evidence that government-sponsored trolls are behind the 'abuse' reports that led to the content takedown," Duy Hoang, an organizer with the pro-democracy group Viet Tan, told BuzzFeed News. "Unfortunately, when activists contacted Facebook, they only received vague responses from the company."
"We are concerned that Facebook is unwittingly helping the Hanoi authorities to censor free expression," Hoang added.
"We have a clear and consistent government request process, which is no different in Vietnam to the rest of the world, and we report the number of pieces of content we restrict for contravening local law in our Transparency Report," a spokesperson for Facebook said when asked about the letter.
The company said it works with civil society groups as well as the government, and that it challenges requests that appear overly broad. It may restrict access to content in countries where the content is deemed illegal, it said.
Hoang said civil society groups like his felt they had a good working relationship with Facebook until representatives from the company, including the head of global policy management, met with Vietnam's Ministry of Communications and Information last April. At that meeting, Vietnamese officials emphasized the problem of "toxic" content on the platform, including content hostile to the state, according to local media reports.
The meeting came after the government pressured local companies to pull ads from Facebook until it addressed the "toxic content" issue, Reuters has reported. At the time, the government said Facebook had agreed to set up a direct channel for Vietnam's communication and information ministry to report objectionable content.
Facebook said the meeting was simply to discuss its policies, products and programs, and that none of its policies changed as a result of the meeting, including the process for governments to request that content be taken down. Representatives also discussed Facebook's assistance to small businesses as well as digital literacy issues, the company said said.
Facebook said then it abided by its own terms of service as well as the law, and that it published requests from governments in an annual report called the Global Government Requests Report.
Vietnam is home to some 55 million Facebook users, a little less than half its population. About 70 percent of people in Vietnam are online.
The open letter, which was signed by 16 activist groups and media organizations as well as 34 well-known Facebook users, says that high-profile citizen journalists were unable to post ahead of and during a high-profile trial of pro-democracy activists from the Brotherhood for Democracy on April 5.
"We appreciate Facebook’s efforts in addressing safety and misinformation concerns online in Vietnam and around the world," the letter says. "Yet 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."