According to the court, Facebook keeps tabs not only on its users, but even on people who have never visited the Facebook website itself.
Facebook must "stop following and recording internet use by people surfing in Belgium until it complies with Belgian privacy laws," the Brussels court said, according to AP.
Facebook must publish the 84-page ruling on its website and display extracts in Belgian newspapers within three months, a court spokesperson said.
"Facebook must also destroy all personal data obtained illegally," the court ordered.
According to the court, besides tracking unregistered users who click "like" or "share" buttons on Facebook pages, even when they have no registered account, Facebook also tracks visitors of roughly 10,000 third-party websites via invisible pixels put on those sites.
An invisible pixel isn't really invisible, per se: it's a 1x1 pixel image file whose HTML "img" tag allows it to embed the code necessary for personalized tracking of users' activities.
Without obtaining the users' valid consent, the court said, Facebook has not only failed to say what kind of information it collects, it does not make clear how it uses that information or how long it stores it.
Facebook has reacted by saying it is going to appeal the court decision.
It said the cookies and pixels it uses are "industry standard technologies," allowing hundreds of thousands of businesses to grow and reach their customers.
The European Consumer Organisation (BEUC) reportedly welcomed the verdict, AP reported.
"This is a big win for internet users who don't want tech companies to monitor every step they make online," BEUC spokesman Johannes Kleis said in a statement.
"What Facebook is doing is against Europe's data protection laws and should be stopped throughout the EU," Kleis added.