The California tech giant acted late Tuesday to block the research from New York University's Ad Observatory Project, citing privacy concerns.

Facebook product management director Mike Clark said the accounts from the project were disabled "to stop unauthorized scraping and protect people's privacy in line with our privacy program."

The NYU project had been at loggerheads for months with Facebook over the program, which used a browser tool to collect data on ads spreading political hoaxes, violence and Covid-19 misinformation.

"Research cannot be the justification for compromising people's privacy," Clark said in a blog post, arguing that the researchers were collecting user names, ads, and links to user profiles even for people who did not install the browser tool or consent to the collection.

But the Facebook move prompted an angry response from researchers and free-speech activists who argued the social network is blocking independent access to its internal tools.

"Over the last several years, we've used this access to uncover systemic flaws in the Facebook Ad Library, to identify misinformation in political ads, including many sowing distrust in our election system, and to study Facebook's apparent amplification of partisan misinformation," said Laura Edelson, the NYU researcher heading the project.

"By suspending our accounts, Facebook has tried to shut down all this work. Facebook has also effectively cut off access to more than two dozen other researchers and journalists who get access to Facebook data through our project, including our work measuring vaccine misinformation."

The row marked the latest clash for Facebook, which has sought to clamp down on third parties with access to private user data while at the same seeking to enable outside researchers to study its inner workings.

Facebook claims it took the action in compliance with a 2019 settlement with US regulators on user privacy in the wake of the Cambridge Analytica scandal in which data was scraped for political ad targeting.

But critics said Facebook needs more transparency.

"We can't allow Facebook to decide what the public gets to know about Facebook. Independent research that respects user privacy is absolutely crucial right now," said Alex Abdo of the Knight First Amendment Institute at Columbia University.

"It's essential to figuring out how disinformation spreads on the platform, how advertisers exploit Facebook's micro-targeting tools, and how Facebook's system of amplification may be pushing us further apart."

Matt Bailey of the writers' free expression group PEN America said the action "is part of a larger pattern of Facebook seeking to undercut or silence anyone analyzing the platforms' practices from the outside."