A Brussels Airline flight to Cameroon had to turn back following a technical problem with the plane.
About 40 passengers had to stay overnight in the airport because they didn't have Belgian visas.
After further delays they began a sit-down protest blockading the international terminal.
Passengers from a flight from Belgium to Cameroon that had to return to Brussels airport because of a technical problem ended up blockading the international terminal after they got stuck there overnight, Aviation24 reported.
About 40 of the 273 passengers on the Brussels Airlines flight did not have a visa for Belgium, which meant they had to spend Wednesday night in the airport.
The chaotic scenes began when they were told on Thursday that the next flight was fully booked, an airport spokesperson told the outlet.
The passengers then began a sit-down protest that blocked access to Terminal B, which handles its international flights.
Aviation24 reported that the protesters were chanting "voyager" (French for "travel") while police tried to calm the situation.
Frank Luntz, a political consultant who witnessed the scenes, tweeted that "thousands of passengers cannot get to their planes, and so flights cannot board."
Another witness, Darren McDermott, described the airport as "an absolute mess" on Thursday. He added that airport staff were taking passengers to the other side of the terminal by bus to reach their gates.
Most passengers on the flight did hold visas and were able to stay in a hotel on Wednesday night. Some of them left on Thursday, but others have been delayed by two days.
A Brussels Airlines spokesperson told Insider that all passengers from the cancelled SN365 will have departed by Friday after an extra flight was put on.
"The passengers without visa, who were not able to reach our provided hotel accommodation, were assisted by Brussels Airlines in obtaining their transit visa with the authorities," the airline added.
That meant the protesters were able to stay in a hotel on Thursday night.
"Some passengers left to their destination on yesterday's flight, the remaining passengers will be able to fly to Douala and Yaoundé today," the airline said on Friday.
Brussels Airport did not immediately respond to a request for comment from Insider.